I paid $4500 for mine with 196,000 miles on it. Have had to fix some vaccuum/idle control lines, put a tire on it (actually, my brother did since he was using it on a trip at the time!), a window switch, new accumulators for the rear suspension, and a tune up. It wasn't maintained all that well, but had a new rear wheel bearing, a new ignition swithc, a valve job, and rear suspension pump replaced, maybe some more, but not bad for 196,000 miles.
Unless it was really butchered, there won't be much wrong with it -- probably bad rear suspension, messy interior, etc, but if the oil got changed at all, nothing else. Probably the most reilable troublefree car MB ever made. Some think the W126 was instead, but I think it is mostly a tie!
Oil pressure at idle hot should be at least 1 bar -- if lower, the engine needs a rebuild -- big dollars, take into account on price. Make sure the oil is normal viscosity, too -- I've seen cars with straight 50 wt in them to mask bearing noise, low oil pressure, etc.
Good luck, hope it is a good car -- we love ours!
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!