I have a 1997 S-500 that I love - the ride the amenites, the stereo etc.
BUT it does have gremlins and parts are very expensive.
The biggest problem that I have found the 140 is the on board computer is not as specific as the newer models...meaning the codes are somewhat general - the CAN BUS system was just starting out, so not as much infrastructure in the car. So diagnosing an intermittant problem can drive you crazy!
The fix to most of the problems is worse - new control modules and they all run $1,000+++.
Find one that has low miles and good service history.
Get it checked out by Mercedes or trusted mechanic first...have them scan for any fault codes and get price to fix those (not just clear them).
The close assist is typically a problem - but simple to fix if you are a little bit mechanically inclined.
The other big 140 problem is the AC. The evaporator starts leaking and then $$$$. Mercedes built the car around the evaporator - so tons of labor to get to it.
good luck!