Hey, Janos. I've not heard of any specific corrosion problems on the newer 210s other than the door seals one.
The 210 does have it's weaknesses (most of which translate to the 208/209 as well). I started a thread a while back discussing some of these:
http://www.benzworld.org/forums/w210...sues-w210.html
You're an MB guy so you may be able to help them with a thorough review of any car they come across, otherwise I am a fan of the prepurchase inspection.
Maintenance records are quite important, be sure the regular stuff has been done plus fluid changeouts (AT!), etc.
While I love the CLK styling, the added room of the 210 is more to my liking. Particularly if you're working under the hood.
Hopefully your parents are ready for significantly higher maintenance bills, too.
Take care and enjoy the ride,
Greg