jc300e is spot on in all his observations. I've had a 400E for 3 years - its been a great car.
A couple of further observations - the M119 is one of the most impressive engines I've ever driven. It has loads of power at any RPM, very refined and from what I've gather on this (and other) boards its a very reliable piece.
I've lusted after a 500E and have test driven a few but I've decided to stick with the 400E- its about 80% as good for way less than half the price.
Some people prefer the 94 and up version of the W124 body. I'm not aware of any other good/bad year issues.
My car also has fairly weak A/C - its not great in stop and go traffic. These cars also have weak motor mounts and as jce300 said I'm going thru a set of brake pads every 15k miles.
A used 400E has got to be one of the best used car values in history. I'm not aware of anything that can approach its level of power, refinement and reliability for less than 10K. Its a great deal.