The things I've found about these cars are:
Sunroof lifting angles crap out easily -- not too bad to fix, but a pain.
Oil leaks at front and rear of head gasket/cover -- some easy, some a real pain. You can sometimes simply re-tighten the rear head bolts to stop the rear leak.
The front flex disk will need to be replaced every 60,000 miles or so.
Transmision pan leaks if imporperly tightened -- easy fix.
Wiring harness trouble -- I'd not buy it if the wiring hadn't been replaced, huge pain.
Pressure accumulators in the self-leveling rear suspension die, causing VERY hard ride. Easy fix.
My 1988 300TE has had the following: Valve job with new valves (160,000 miles), rear wheel bearing, ignition lock, power steeering/leveling pump rebuilt, door lock actuator replaced, window regulator replaced, one set of shocks (I think), numerous sets of tires and brakes/rotors, a couple of switches, a mirror and turn signal (accident damage). Otherwise, not too much service.
Still running fine at 200,000 miles, uses a quart of oil every 2000 miles or so, looks like it will run forever for my mother!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!