I have a 94 S320. I had the same concerns you did regarding lack of acceleration. I looked for an S420 for the longest time until I found this S320 that was too cheap to pass up on. I stole it for less than half the blue book. It was a little rough, and there are a few things that you need to know before you buy one...the most important being the AC system. Make sure that the evaporator has been replaced, because although you can get the part rather inexpensively(200-600) depending on where you get it, it is in the most inconvenient place...and requires between 17-25 hours labor to replace. When I got mine, I knew the AC system was shot, so I had figured around 2500 to fix this problem....I got lucky, and have a mercedes mechanic that does work for me, and he basically is fixing mine for free(just parts). The other things that are known to be problems on the w140 chassis are the wiring harness, and the vacuum pump. I also have problems with the retracting mechanism on the seat belts...Most of these problems are minor and inconvenient...as the AC problem has been, but the engine and transmission are bulletproof. I upgraded from a C class, and I would never go back. I drove an E class 97...and have to say that the w140 92-99 S class is by far the best mercedes I have ever driven or owned....despite the problems mentioned above.

__________________
1994 S320
131K miles

1998 ML 320
126K miles

1994 Blue C220
122K miles

Im going to Grad School @ the University of Iowa
Go Hawkeyes!!!