Look for a low mileage 1990 or 1991 (the latter has ASR, both have ABS and air bags) 560SEC if you want the best build quality and reasonable maintenance costs in a modern MB coupe without nightmarish electronic complexity.
specializes in these.
By 1990 the 10 year old W126 design was pretty thoroughly debugged and the few problems are well known and addessed extensively on this board.
Sadly, the American Muscle Car derivative styling of the SEC does not compare with the earlier SLC, but it is a much more modern and reliable vehicle.
You can buy six or seven of these for one new SL.