I know about several 210s who have had the roof replaced due to rust as well.
My parents had a ´98 280 4MATIC and a ´00 240, the thing that scared me the most was how extremely fast you get "worms in the paint" in places where the paint get scratched. Seems that the steel quality is really bad on these cars.
They have a W220 S320 now, and I recently discovered lots of rust in the rear wheel arches as well as on the trunk lid. The "worms" also applies to the W220.

I think I´ll stick to my W140 400 SE for a while. The W140 tend to rust too, but not even close to the W210 and W220.

-Michael