It's mostly a function of how much time the car spent in the Rust Belt. Our 1998 car has been in the south nearly its entire life and there is absolutely no rust on the car apart from a small area where some sub-standard body work was done. The underside is very clean, too. I'm quite certain the exhaust is all original apart from the oxidation catalyst.

Two neighbors have W210 cars. One is an early wagon and it carries some serious rust. Not sure how much longer they'll make it thru inspection. The other is a face lifted E430 and that car is real clean. Can't see any rust on it and it sits outside 24/7.

We had an '02 wagon. Got rid of it at 140k. Ran great but the rust was making a complete nuisance of itself...popping up everywhere. Kinda like whack-a-mole. I couldn't keep up with it. MBz repaired some minor stuff twice.