As far as chassis types, I think you can't go wrong with a well-maintained W116, W123, or the first couple years of W126 cars. This would include the late 70s/early 80s 300SD, 300D, 300TD (wagon), 300CD (2-door coupe), etc.
Look here for lots of great pictures of different models/chassis type, etc.: http://www.dhc.net/~pmhack/mercedes/MODELS.HTM
The 116/123/early126 cars have the bullet-proof 5-cylinder all-iron diesel, with turbocharger (except pre-82 300D/TD. Turbo is very desirable for much better power), and the suspension/transmission/etc. are all very well done.
I'd avoid some of the later diesels with the aluminum head...It easily warps or cracks when overheated. Also I'm not crazy about the later cars with 6-cyl. diesels, and I've heard MANY horror stories about the 350 SDL, so I'd stay away from that for sure.
Before I make any of you 240D guys mad, let me add this....
240Ds are also excellent cars, and possibly even MORE durable, but with the 4-cylinder diesel they can be unbearably slow, especially with an automatic transmission.
I hope this is helpful information...I'm sure I won't be the only one to chime in with advice and preferences.