"Have you any preferance to any model"
Well, I have two 124s (1991, 1996) and a 126 (1992). I love 'em in spite of the fact that they now all have corrosion starting on their front wings. The 126 range is also prone to corrosion under the rear window; I checked that before buying mine and it had obviously been fixed, and it's still good four years on.
As suggested above, the R129 SL seems relatively rust free, although I have seen reports about drainage problems and doors, if I recall correctly. I've wondered out loud before whether SLs get special care in the manufacturing process, given their historical status as MB top model. I do like that model, but it's not for me on practicality grounds. Also, as suggested above, I think the W201 is pretty sound. As to earlier models, I'm not sure that the W123 has a particularly good reputation corrosion-wise.
I guess we all have to face the fact that any old car is going to rust, although my Audi 100 had but one pinpoint of rust after ten years of use from new, and my Golf VR6 was similarly sound at the same age (yes, I choose my cars carefully and keep them forever). However, the prone areas in MBs are all well known and the repairs can be budgeted for and knocked off any asking price.