Slow down! too many posts too soon!
1. Gearbox. toyota w55 & w57 gearboxes have been used here in oz behind very potent LS series engines and the earlier locally built 308 holden (buick based) and 302W 351W 351C ford engines. and not stockers, but big hp versions. and they hang together easy. so your idea of needing a hugeass box behind an m117 is BS.
Just get something commonly available, which can handle the lb-ft rating. You need compact, because the trans tunnel taper is very steep, look at the current auto and any mercedes manual box. they are extremely compact & efficiently designed. So you need to follow the same thought process.
Forget about all the boy racer talk. You need not concern yourself with scrub radius or hub loading. You will never be able to safely approach any corner or apply enough braking force through the vehicle to get anywhere near the structural failure limits (assume, for this purpose, that the vehicle has new / refurbished standard OEM parts).
If you're a dickhead about it, then yes, you will cause yourself issues. Fitting stupid offsets and widths will assist you in doing this.
Stick to the factory offsets & AMG offerings. Remember, they needed to be approved by the TüV before they could be fitted - for a good reason. So your absolute limit, is a 17x8.5 ET18 front with 235/45ZR17 91W , and a 17x10 ET17 rear with 275/40ZR17 91W load rated tyres. In the US, these are easily obtained. Not so in other markets, due to that rear size (camaro SS).
What's funny, is that you will need a fire breathing monster to overcome the chassis dynamics and adhesion limit of R spec rubber in those sizes. That in itself means the car will be totally unusable on the street. If you want to do that, then forget everything, go back to Square #1 and setup the car for track only use, with full rollcage and custom made subframe / chassis components. there will only remain an outer shell of a mercedes. It won't be a Mercedes from anyone's perspective in anything other than shape. (and by that time, all your panels should be composite and glued on, nascar style).
If you truly want a sensible performance car, with all the refinements Mercedes-Benz applied in building the standard vehicle, then stick to originality.
A nice set of period-correct rims, and some small modifications to the cars injection setup are more than enough to give you a very nice, comfortable car to cruise in, to be seen in and admire.
Going too far is very easy, and will ruin aesthetics, driveability, comfort and any value - real or perceived.
If you want performance out of an engine, then you need the AMG cams, or the Feind ones, or a couple of producers in the fatherland who still make them. your best bet is to stick with the hydraulic cams, as the older solid rocker pivots are almost impossible to source, are prohibitively expensive and need frequent adjustment to keep valve lash correct.
the ECE / RüF versions of the 500 and 560 engine differ in only 2 areas: 1. Camshafts (slight change to lobe centreline angle and duration) 2. Static compression (10:1)
Cams are still available, and compression was solved with flat top pistons instead of the standard dished ones. Simple as that. And your engine was good for 220kW factory. Fuel metering on the KE can be adjusted at the EHA on a setup which is not requiring overhaul.
You can also apply the same logic to the 4.5 D-jet engine. but it will require a little more work, that's all. IMO the 5.0 alloy engine is the best offering.
forget about porting & polishing. big waste of time. these things flow massive amounts in the standard configuration. Sauber raced the m117 years ago, and the modifications to the heads were surprisingly small. consequently it is simply not worth spending huge $ investment on something which will net you stuff all except wanker bragging rights "my heads ported & polished" woop-dee-doo. I've seen standard cars flog the snot out of modified ones on the street. So, unless you've got deep pockets and youre racing, forget it.
These cars, when appropriately tuned & modifications kept within the ethos of the original MB-engineered solutions will provide a very safe and to use your words "docile" car on the limit.
You'll get plenty of warning when the cars about to let go, and power oversteer is easy to produce, when the auto is in good nick and shifting properly, and the suspension is also in good condition. If the car needs suspension or transmission work, then the predictability at the limit is not a finite thing, and recovery from a big tail-out moment at speed is not easy with a sluggish badly performing worn out automatic. Just like being one gear too high in a manual, the throttle response will not be sufficient to get you out of jail if you land yourself in there.
So, if you drive sensibly, of course, you'll never reach that limit.
Even if you don't drive sensibly, then the limits of adhesion of the standard vehicle, with good quality rubber, are very high and very predictable.
Now, I'll give you my suggestions as to what's worth doing, from a handling & performance perspective.
1. Springs & dampers. H&R do make springs. Very hard to get. but they make them (still available) What you will not find however, are good dampers. You will need to get some custom made Bilsteins. That will cost you $$$ and will completely bugger the comfortable ride of an SLC. An SL was never 'comfortable' as the shorter wheelbase even on standard suspension is like jello wrestling. looks like fun, until you're in it.
You can, for cosmetic appearance reasons, take your standard springs and remove one coil from the front and one half coil from the rear springs to lower the ride. Make sure you use new spring pads. some extra adjustment can be made on the pads themselves (numbering system) but if you just want to get an 'idea' then you can safely cut the springs with a cutoff wheel on a 4" angle grinder, without creating a tempering issue on the coil.
Careful measurement and a quick cut is necessary when doing this. Measurement is critical, and you only have one chance to cut, you can't cut again or shave mm after a rough cut, as that will create a tempering problem. It is a good method for setting a ride height estimate, if you wish to have aftermarket springs made to a custom rate.
Do not use heat or a hacksaw. both those methods cause problems (temper and cracking / fatigue).
2. Sway (anti-roll) bars. Feind seems to be the only source at the moment. are they worth it? YES. even on an otherwise totally standard car, the absense of extreme body roll in really hard driving is much more satisfying and makes for a more comfortable ride at the limit.
I mentioned this in another thread some time ago. replace the rear subframe with genuine bushes. That includes the diff mount as well. On the front, you need something that's a little better than the crap that febi produce. the german 107 SL website guys have an OEM front subframe bush, filled with potted urethane. That's the start.
You should make sure your upper and lower control arm bushes are less than 5 years old. They're not made like they used to be, and as a consequence of cheapass manufacturing practices of the modern era, we're stuck with shit quality rubber bushes by OEM's that last 3-5 years max. after that the compounds degrade and the shore hardness drops right off, to the point where you'd be better off using a sponge in there instead.
Custom cold cured polyurethane with an 83C hardness is absolutely a good idea on the control arms. Not only does it liven up the steering feel, it provides a much more direct tracking feeling from the road. The tradeoff is a tiny increase in NVH levels, and a little more 'feedback' through the steering.
3. Balljoints, tierods, pitman arms, draglinks & steering damper.
replace all of it with new. Use the Lemförder stuff if you can. It seems to be the best quality from all the OEMs, with a reasonable price.
4. Rear end & brakes: Rear brakes are the same for nearly every MB model from 1972-1995-
There are a couple of exceptions: 124 wagons & the C36 come to mind.
Basically, there is nothing wrong with the rear brakes at all. theyre perfect for the application. Rebuild the callipers and use decent pads (OE Ate pads are good but produce dust - a byproduct of a good pad that doesn't fade) and don't forget to rebuild the handbrake as well (new shoes / springs) while you're at it.
the rear wheel bearings are a bugger on the aluminium arms (semi-floating hub), so make sure they're in good condition. R&R of those is a PITA job and time consuming. The 280/350 (early) SL/C's had the one piece rear trailing arm with a more conventional antiroll bar, and this is what the Factory rally cars used. I'm not saying it was a better setup - just that the factory rally cars used that particular setup. whether it was because of simplified maintenance or reliability or improved handling is hard to say. I ripped mine out of Lurch and replaced it with the later anti-dive setup from my 560SEL, and added a new rear antiroll bar.
5: Front brakes
the best solution for a 107, short of a full-on stop-tech upgrade front & rear, is to use the 560 master vac & cylinder, with the w126 560 callipers & rotor.
Why the 126? well, they were designed to haul up a hugeass 126 for starters.
Secondly, the rotor, while the same diameter, is much thicker, resulting in better heat dissipation. The pads have much more surface area, and a twin 57mm front piston is never going to fade. Ive tried many times, and even on a track I've blued my rotors but never had fade.
Why not the later model 107 4 piston callipers? well it's simple really. they "ain't all that". Combined with the need to use the late model control arms (560SL has a wider front track to all pre 09/85 107's) the need to use the modified spindle and then the different rotor and fixed 4 piston calliper, it's a case of unless you have the complete 560Sl front end at your disposal, it's a complete WOFTAM.
THEN you have the added problem of finding aftermarket rims to clear the callipers. Gen 1 pentas will not fit, because of calliper clearance problems. Lorinser LO's will not fit - no biggie if these rims aren't your thing, but in terms of styling, these two period-correct rims go a LONG way in getting the nod of approval from those of us who have 107 in our DNA.
So it comes back full-circle, to Rims & tyres. Like everything Mercedes-Benz does, they engineer a complete package. It works in perfect synergy. Everything about these cars is symbiotic. You cannot change one thing without it affecting something else negatively. So any changes you make need to be wholesale and within the same framework that MB engineered their solutions.
You will find it very difficult to improve on an original vehicle in 'collector' condition. I'm of the opinion that everyone who wants to modify a 107, should at least drive an unmodified one that's in concours condition for a few hours. That way the person gains a reasonable understanding of the already good vehicle dynamics and performance.
Yes, by modern standards they are lacking in some handling finesse - but they make up for that in comfort and driveability and enjoyment. What's more - simply bolting on one of the period-correct wheel & tyre combinations from AMG / Lorinser / König etc will provide the 107 with a much more 'sure footed' feeling on the road, than the old 14" steelies, bundts or the 15" 15 hole alloys ever could.
Lastly, some blasphemy. Replace the steering wheel from that huge boat tiller, to a smaller 350mm Momo or Nardi. Or an AMG italvolanti 370mm wheel if you can find one. That alone makes a huge difference to the driving feel. It does however lend itself to a bit of 'instrument obscurity', but it's easily managed by altering the driving position or using a GPS / satnav.
So that's my long-winded and highly opinionated version of what's reasonable & prudent on a 107. I'm sure there will be plenty who disagree in part or entirely, but since I'm speaking from actual practical experience from modifying both my SL & SLC and having the luxury of having a very early model and a very late one, I can see where the factory made the 'needed' improvements. I'm also only stating from experience what I have found to work and what was a waste of time.
I'm not saying that everything I've done has been successful either. a lot of my findings were trial and error, but I've always made an educated decision and done my research before attempting, to ascertain the viability of modification.
I suggest you do the same, find out what works for you, what you would like to do and then research it thoroughly, before committing to the modification. Don't forget to make it reversible if possible, as this guarantees a fallback option if you are dissatisfied, or need to baseline reference your modification in case you need to alter it more than once.
hope that all helps somewhat.