The only problems of note I had with this car was the body. It just caught a case of cancer in the Northeast climates and slowly died. The rocker panels and the structure along the bottom of the car just dissolved in 16 years. The car was "put to sleep" with a very strong engine. It was cheaper to replace it with a used 1982 240D than go through another body repair job.
I ordered the car with next to no options (no a/c, manual transmission, and no electric anything - only a sunroof as an option) so I had a very accessible engine and learned to do Diesel maintenance over the years. Not much to it for the most part. The newer cars (W123 from 1977 on) have some very nice features for the do it yourselfer (the oil filter in the 1975 model is a real challenge to get to, and someday I would like to meet the guy who thought that was good place for it - inside the front suspension arms on the driver side), and addressed the rocker panel rust by just eliminating the rocker panel altogether. They also improved the corrosion resistance of the car significantly with that model.
So, if you get a car that has not spent much of its life in a climate where it snows and the locals use a lot of salt, the car should prove to be mechanically reliable. Steering play and suspension bushings and any rubber parts are likely to be starting to fail if they have not already, just from time, but that is to be expected with a car that is over twenty five years old. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)