A lot of people say the air conditioning evaporators are a problem with this car, but I don't have exact figures. Secondly, the head gasket and valve seals on the gas engines are problematic. I guess the earlier models had seals that didn't hold up, which causes the obvious problems, and the heads need to be rebuilt. If the head doesn't go first, the headgasket usually starts leaking oil out the sides.
Bottom line, do what I did and buy a car that has documentation to show the head has been rebuilt and the headgasket replaced.
Another option is the w124 diesel. Now that's a nice ride!
'84 300D, 173k
'90 300CE, 162k