Between my mother and myself, we have six V12 W140's and R129s. They are very reliable if you treat them properly and stay ahead on maintenance. You've got the middle model--which I like much better than the '96+ 140s--that can have wiring issues. Have the top engine harness replaced. I've got a source for them at a reasonable $750. From the stealership they aren't too much more.
The suspension is one of the worst systems on your car from a cost perspective. It's right about $4k in parts to replace shocks and springs. Front shocks are almost $1k and rears are about $700. A cheaper solution is to loop the SLS line and run with standard Bilsteins. You'll need the 23mm rear spring shims (most likely) to have the car sit level. That costs about $500 for all four shocks. Much cheaper than OEM.
Do yourself a huge favor and change the serpentine belt, fan clutch, radiator, thermostat, and water pump. It's not really that hard if you're a DIY-er. If not, get it done at a good Indy shop. Don't let dealers muck around with your car unless you're very close with them. It takes a special person to deal with these M120 engines. Additionally, use the search to look up the "cool harness." Instead of that route, I've installed SPAL electric fans in all of my race cars, Mercedes, BMW's, etc. Everything except a few Italian exotics and my F350 monster have SPAL fans and I swear by them. Most people think it's too hard, too expensive, etc... well it's BS. It'll drop your engine temps by about 5 degrees celsius if not more. At least that's what I've found out.
How many miles are on your S600? Is it a five speed or four speed tranny? How many owners since new? Where was it located before you bought it?
Enjoy and welcome aboard!
One more thing, run ONLY Mobil 1 oil and change every 3k miles on the dot. I change my oil every 1000-2900 miles. My mother changes hers every 2500-3k. We use Mobil 1 0W40 and Michelin tires. Nothing else for our cars! Another little thing I do, and at this point you probably think I'm crazy but what the hell, is I only fill up my gas when it's cool outside, there are no other cars filling up, and only using Shell premium.
thanks for your detailed overview. my S600: a few days ago i purchased a beautiful '95 S600 ($7200, San Diego Lexus), 123K miles, 2 owners, calif reg'g/domiciled since new. got orig complete books, manuals and sticker/ invoice. moonstone gray paint, parchment interior. $134,170...out the door, incl $3,000 guzzler tax! LoL. when i get the car back i'll post some pics here.
i believe its the 4-speed tranny. i do my own maintenance. yes, i had previously read about the S600 engine wiring harness reliability problems, insulation breakdown. i have not inspected mine yet but already prepared to replace it if needed. who is your source for the $750 engine harness? is it a sub-section of the engine compartment wiring or the entire engine compartment, soup-to-nuts?
she test drove just fine prior to the purchase. while the salesman followed me home saturday in the S600 he noticed the trans intermittently down-shifting from 4th gear to 3rd gear while cruising on the HWY. anyway he was honest enough to point this out to me when we arrived at my home and immediately called their indy shop to pick up the car for repairs.
having done quite a bit of maintenance on my 280E, 300E and 300D, i figure the S600 trans problem couldn't be that serious. i'm thinking something to do with the shift linkage/bushings, trans vacuum modulator or throttle linkage/sensor to the transmission. whatdaya guys think? prior to sale the dealership replaced the engine oil pan and trans oil pan gaskets. perhaps they didn't properly reassemble the trans linkages. a member on another forum suggested looking for a faulty kick-down switch, solenoid or linkage to the trans.
apparently the rear suspension accumulators are shot as well. rear suspension very stiff, no floating for this teutonic yacht. too bad i just had surgery on my shoulder last week so mechanically i'm benched for a few more weeks.
good advice on the mobil-1 synthetic. i was not sure if it would be a good idea to run synthetic on one of these motors +100k miles.
i would really like to get the MB dealership VIN maintenance history dump, as i was told the previous owners had it serviced locally there. from my research it doesn't look like they will give it to me, although i have not asked yet, and i don't see any forum members doing favors in that regard.