Greetings from Raleigh!
I have a 1992 400e, basically the same car as the 94.
I purchased mine last year with only 63k miles on it.
From experience and what others have written on this forum, here are a few things that are possible problem areas on a 400e/e420.
Power steering leaks. May be just a hose or worse. I recently had my p/s pump and steering rack rebuild due to leaks.
Engine harness. The wires tend to crack and cause a bunch of different type of electircal problems. I had to replace mine when I was in CA. The dealer was not willing to cover any of the costs. You may get lucky and have a dealer cover it under good faith. The harness set me back somthing like $500.
Engine cooling issues. Some report of running hot when sitting in traffic. I only see this on very hot days. Other than that, no cooling issues.
Transmission issues. Slow to engage from R to D. This appears to be a design issue in the transmission. I have this problem and just live with it. Not too bad of a delay, just anoying at times.
Since the car you are looking at is at 100k miles, I heard that you MAY want to have the timing chain replaced.
As rpalens said, a good mechanic, service records, and CarFax are a good starting point. These are just a few areas where I would concentrate on. Do a search under 400e and e420, you will see what others have reported as well.
Don't get me wrong, the 400e/e420 is an AWSOME car. I love mine. It looks and drives like it is a brand new car. As for the power/performance.... well, I still have not been beaten on the highway. I have raced M3s, 540is, Lexus SC400, oh yeah, and a few Pontiac Bonneville SSie (or whatever they are called. Seems drivers of these cars think they are fast.) OK, I did get beat once, but not by much. It was a 430CLK.