Check to see if it has the ASR (anti-slip) option. Luckily, my old 93 400E did not have the option and I had no problems with it; but others have reported that the electronic throttle actuator (ETA) seems to go bad sooner on the cars with ASR. A new ETA is about $1,200 from the dealer.
I think one of the funniest posts I've read on this forum was when someone once asked what it meant when the ASR light was on. Deanyel replied something to the effect that it means the car has ASR and you should sell it immediately.
If you turn the key on and you see the ASR light illuminate on startup, you will know if it has this option.
Also, as the 119 engine ages, there is a code 19 that you may get on the diagnostic module and it means you have reached the limit of electronic adaptation correction. This is an emission issue and causes the check engine light to illuminate without your being able to reset it. It can be corrected with a chip that is discussed about 1/4 of the way down this page. http://www.k6jrf.com/MB_S500.html
Good luck.