The milage seems a little low--probably the dealer wants more for the low milage, but it indicates the car may have been sitting. In such case, expect to replace the rubber suspension components and look over the fuel system components carefully.
I will leave it to others to comment on what other classic issues there are and what they think a fair price is. Cheers, Andrew Seidel.
edit: Actually, I do have some comments on MB diesel classic problems:
1) leaking oil seal on the tube that carries oil from the block to the turbo -- consumes a lot of oil and leads to turbo failure -- replacement turbos run around $800 plus installation
2) self-leveling suspension -- pump and valve rarely fail, struts will eventually fail if the nitrogen accumulators are not replaced at the proper ~100k intervals. A bouncy ride indicates the accumulators are getting low on Nitrogen, a hard/dead ride indicates they have already collapsed and struts may have been damaged -- look for oil leaking from struts. Replacement struts are ~$400 per side but accumulators cost ~$100 per side
3) climate control -- typical A/C problems may exist, but it common for the A/C computer (pushbutton unit in the dash) to be bad from thermal expansion and solder defects. Rebuilt units run ~$300, but tend to last longer than the originals with today's better PCB technology
4) cruise control -- usually the computer goes out, similar to the climate control. In the rare case, the actuator motor will be bad. Rebuilt units are ~$300 for the computer, >$200 for the actuator
5) hair-line cracked fuses -- sometimes the barrell-shaped fuses will crack around the point-of-contact with the tabs and go open-circuit even though they look OK -- best to just replace them all upon purchase of the car to avoid unexplained electrical gremlins
6) window switches and glass lifters are the next most likely things to go. Lifters run ~$100 per side, switches run ~$50 a piece
7) weather seals, clogged drains can lead to floor rust if leaks were not promptly addressed
8) suspension wear -- general slop develops from the rubber components wearing out. Suspension parts are inexpensive, though the labor may be significant. Rebuilding things like bushings, sub-frame mounts, ball joints, steering-damper, transmission mounts, engine mounts, and differential mounts will tighten the chassis up with great satisfaction -- alignment after the repairs is one thing only the dealer seems to get right, otherwise most other things mentioned above are DIY repairs
9) radiator necks are made of plastic and tend to break -- the radiator should probably be changed every 100K miles, anyway -- new replacement Behr radiators have a reinforced neck to avoid this problem. ~$300, I believe.
85 500SEC Lorinser
83 300D Turbo