I bought my first Mercedes, an '86 300E with 93,000 miles, about one month ago. I've been in the process of fixing several problems and have most of them licked now. As each day goes by, I enjoy driving this car more and more, and the deeper I get into fixing it, the more I'm impressed with the quality of engineering and workmanship that went into making it. My car feels, no kidding, just like a new car in the way it rides and handles, even the way it feels and smells on the inside. It has loads more power than your 300D and is the type of car you could drive all day at way more than legal speeds and enjoy every minute of it (except maybe the tickets). I plan on taking it on a 3-day, 1400 mi. TSD rally in the near future, right after I get back from towing a tent-trailer and the family from L.A. to Canada and back, and I have absolutely no qualms about doing so.
As long as it's in good shape (have it checked by a reputable M-B mechanic) just buy it, fix the obvious problems and never look back!