You don't have to take it to a Mercedes mechanic all the time. I took my gas Mercedes to a Goodyear to be worked on, my diesel to an auto cool place, for brakes to a general mechanic (turns out I would have been better off taking it to a specialist that time, due to an intermittent brake problem that he wrongly diagnosed, cost me about $400 and the problem still exists. From members on this site, I think I've zero'd in on the problem, a $40 part).
I think once you can get the Mercedes up to where it should be as far as maintenence and stuff, then you should be ok. Obviously, when you get to higher miles, things will have to be repaired or replaced like on all cars. For example, the transmissions are supposed to last til about 225-250k, then they have to be rebuilt.
Aside from major repairs, on my diesel, the repairs aren't that bad looking back on them, but going in when you don't know what to do, it sort of sucks. For example, look at 2 DIY posts. The one is at the top is mine, the one underneath from someone else. I saw his DIY before the repair, but it seemed to be too much work, and for me, not explained as well as I would have liked (talking about something AFTER you've done the repair, sure, DIY's seems logical, but prior to doing them, many instructions seems confusing). View this....
http://www.peachparts.com/Wikka/W124ShiftBushings
So, bottom line, it sucks when something needs done. After you "master" that repair, you think, that wasn't so bad.
Good luck in whatever you decide.
Jeff 1991 300d, 102k