Okay, next time you're going to get a mechanic's report and perform due diligence on the records BEFORE the purchase, right? But if the car drives and looks as well as you describe, maybe you've gotten lucky.
This leaves you really with two separate issues:
- Mechanical. Sounds good initially; you might want to have a Mercedes expert give it the once-over, for planning purposes if nothing else.
- Consumer. First, verify beyond all doubt in your mind how many miles the car has. Then decide whether you feel that the car was badly misrepresented to you, and if so, what your state's laws have to say regarding odometer disclosure, cooling-off periods on used-car purchases, and other relevant matters.
If I interpret your message correctly, the car could have almost half a million miles on it! It is not unknown for these cars to go a long, long time -- if they have been conservatively driven and meticulously maintained, and perhaps if it started out as a right-hand outlier from the spread of the quality statistics. It's also very possible that significant work has been done to major systems, making them effectively younger (perhaps a lot younger) than the chassis. Thus the importance of having the car checked over by someone who really knows the marque in addition to knowing general mechanical principles.
This applies, but much less alarmingly, to a more conservative interpretation of your message -- i.e., it's supposed to be 205k but it's really 250.
You might also want to follow alt.autos.mercedes and look for mailing lists for the model, just to keep up with what its issues are and how best to handle them. It's a complicated car with a few years and miles on it (at least!), so things *will* come up, some of them just annoying and some of them significant.
Best case, you got a really nice and gracefully aging car in a fair deal. Worst case, you got ripped and need to decide what to do about that. Some of this is based on objective facts and some of it on how you feel about the whole thing.
Best of luck,
--Joe