My 300TE has the normal front cover leak (will have to fix that sometime) and has had the head rebuilt (PO -- about 160000 miles). Other than some fuel delivery trouble, nothing else.
With good oil and proper change intervals, I'd expect the bottom of the engine to last at least 500,000 miles without work. Probably going to have to do head work once or twice, nothing lasts forever.
Older ones had valve guide wear problems, the replacement guides are harder, shouldn't recur.
Bodywise, watch out for rust at the jack points (need to fix this on the 300 TE, passenger front), the rear suspension links will fail, the replacements are from the 210 chassis and will last longer, normal AC problems. Fuel distrubutor will eventually go, cost big $ these days for some reason.
Else pretty maintenance free.
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!