I would not rule out the '86-87 300SDL. I ALMOST bought one 3 years ago (and sometimes still kick myself that I didn't!) It has the 603 engine, which I believe is a very nice engine, if not exactly as bulletproof as the 617. (On the other hand, it has hydraulic lifters and thus doesn't need regular valve adjustments.) The two main issues with the SDL of that vintage are 1) trap oxidizer and 2)potential for warped or cracked heads. RE: the trap oxidizer, this should have already been removed by a previous owner under warranty (easy to check: look for a coffee-can-sized and shaped cylinder on the right side exhause manifold), and the head issue affected a minority of these vehicles.
Best to avoid the 350SDL for reasons mentioned in earlier post (the rod-bending problem did not exist for the 300SDL).
I agree with Neil above that the late-model turbo diesels are VERY VERY nice, although if you're looking for something you can DIY on, the W210 diesels are probably not for you. The older diesels are basically mechanically controlled, whereas the new ones have very sophisticated electronic engine management systems.