Can you tell a gas engine model from a diesel by the cars model # if the ad doesn't specify which kind of engine the car has?
D in the model, ie 300D means diesel for mercedes. Model decals on the car itself can be altered, check the title for model confirmation and match the VIN for carfax.com
Right, most common models found in the us since the late '70's:
190D(2.2 non turbo and 2.5 turbo)
240D (no turbo = no go)
300D (early years may not be turbo, the legend IMHO)
300CD(2 door, early years not turbo)
300SD (big model)
300SDL (even bigger)
the above cars made the MB legend thru about 85 I think, I can't remember exactly when engines changed.
300D (newer model (late 80's, early 90's, not the older engine, not a legend)
350SD (at your own risk)
E300 (late 90's)
E320 CDI (current model, pricey and an entirely different level of tech)
I may be way off base with any of the above info, but this should be a start...