I want to thank all of the respondents to this thread for their advice.

It turns out that my 300D, currently at an auto body shop for repair after being rear-ended, only has surface rust along the windshield. The man who originally eyeballed it, and predicted deeper, endemic rust, has pulled away from his original, gloomy appraisal.

In the meantime I heard about a place called CarNation, very near to where I live, which is supposed to be quite good. I happen to read about this place in a magazine called Car Craft. Apparently the man who runs the place is a real pro and enthusiast. He uses DuPont paint rather than the two, more common German Mercedes paints.

Peter4