autohausaz.com is, yes, a good place for parts. I like to go between two other sites as well and pick the lowest price
as far as miles and "general wear" items, it's all up to you. I've had the same 1994 C280 since 14k miles and I'm about to hit 240k. Yeah, I had to fix the headgasket and wiring harness years ago, and my headliner is drooping (i've seen 1 other c280 with the screwy headliner), but I don't foresee the thing lasting any less than at least another 200k. i'm on the factory original
lower control arm bushings and shocks (as well as all steering components), starter, alternator, both vac pumps, etc. only things i've had to replace were water pump and radiator (due to a deer hit), MAF, o2 sensors, fuel filter, ignition lock tumbler, and a belt tensioner. upon replacing the belt tensioner, I discovered it wasn't even broken. i just didn't loosen the pivot bolt to allow the tensioner to do it's job. went ahead and put the new one on anyway, and kept the old one as a spare.
also, an oil change every 20k miles with mobil 1 0w40.
though I am sure ther are a few c280's out there that just didn't cut the mustard and 250k was pretty much all they could stand.
250k is average miles for these cars at this point, but that's just because there are few of them out there that people have driven past this point.
over at CLUB202.COM
(forum is down for now) there was a member 2 or 3 years ago who had 560k miles on his 1994 or 1995 C280. good compression, good power, and still had plenty of life left in it.
so "too many" miles all depends on you and the car.