Well I'm glad we agree on the suspension front
There is no front subframe per say, the suspension components tie directly to the chassis. I wouldn't worry too much about chassis stiffness, a lot of this car has roots from the W201 (which was a successful DTM chassis). If you ever get to tear this car apart, you will notice it's already reinforced with lots of HSLA steel, on top of it already being a very solid construction.
Brakes wise, notice that your front brakes already have cooling ducts, I don't think the rear ones will need any additional direct cooling, especially not if you consider dropping in E55 brakes. If you can get a BBK from wilwood, then well, awesome! I saw that brembo has a mad BBK for the w202, but unfortunately they are too expensive for my current student budget.
Excellent choice on the BBS rims, I totally would love to put BBS on my car, but they can get quite pricey
(I've got RTX rims now, they are pretty nice).
I'm getting a little jealous here
You've got all the resources I wish I had, I'd be building the most awesome MBs everyday
If I were to toss in my recommendation, there are 3 basic m119 engines, 4.2, 5.0, 6.0, but GOOD LUCK trying to find a 6.0. It's extremely rare, found on basically the E60 AMG and the CLK LM (both extremely rare cars). The 5.0 and 4.2 are both pretty common and easily found, with parts available for cheap. I'd go for the 5.0, I know turbos are good fun at making a car fast, but there's no replacement for displacement right? (Plus they both have the same reliability on average). A 5.0 twin turbo in a 22 year old car, eating any supercar that comes your way?
That sounds totally awesome.
Transmission-wise, if you want to go automatic, I'd use a 722.6 with a standalone TCM. During the Daimler-Chrysler partnership, the 722.6 (also called the 5Gtronic) was used in chrysler vehicles as the NAG1. The drag racing scene in USA quickly figured that with little modification (mainly running the euro-spec valves in the valve body), the NAG1/722.6 could handle 1000lb-ft of torque like a picnic. There is a standalone TCM available for it, letting it work in any car, and it would be an easy fit in the W202 (actually, the W202 used the 722.6 starting 1997 on all models). Bell housings and flex-plates won't be an issue either.
If you want to go manual, ask the local south-african boys here
They will probably know more about this than I will, I find automatic transmissions way too fascinating to be bothered with manuals. The only thing I know is that there is a massive electronic issue with doing an auto -> manual conversion if you use the stock ECM, but you're dodging the chevrolet corvair ou of that bullet.
Weight balance-wise, notice that the engineers at mercedes thought about you 22 years ago, the big heavy battery is at the back
A good portion of the engine is behind the front axle, and the transmission is further back too (unlike transverse engined econoboxes), the weight distribution is probably already fairly close to 50/50. I don't see how much you will improve over here.
If by some twist of faith I end up in wisconsin, I'd be glad to lend a hand