From the vantage point of 2010 I think the best 124 is the newest, best cared for E320 you can find. It needs to have records showing the head gasket and wiring harness have been replaced, and perhaps the electronic throttle. They are very solid cars once this work has been done.

Other occasional problem areas are common across all 124 cars - AC evaporators, for example.

One item I haven't seen mentioned. Starting with 1994 cars MB went to water based paint systems. I think the 1993 and earlier paints are more durable, especially in non-clearcoat colors (e.g. white, black). My wife drives a white 1993 300E 2.8 and the paint still looks new. It's really amazing how well the car is holding up with age.

I don't have direct experience with a 124 400E or 500E, but think they tend to be more expensive to own. Not necessarily more problematic, but use more expensive parts and/or are more labor intensive to work under the hood. That big V8 is stuffed in an engine bay originally designed only for a straight six and is very tightly packaged. I would expect more heat/age related problems in the V8 cars as a result, such as brittle plastic bits, squeeky serpentine belts, or aged radiator hoses.