Those 98-99's are pretty good cars, here's a few problems that come to mind:
I've seen some cars of this era, especially the 97 E420's and the Diesels of all those years, break off spring mounting perches (on the subframe assembly), need to get a body shop to install a new one. Not super-common, but I've seen it more than once. Also for some reason I've seen a few Diesels around this era have a seized front wheel bearing, resulting in needing a new front steering knuckle. No idea why I've only seen this on Diesels.
Rear axle seals on 202's and 210's I've noticed you will find leaking or at least seeping oil, the seals of course are replaceable.
Other than that, hmmmmmmm, well all 210's are kinda noted for weak rear window regulators.
Also on 98-99's you have that smartkey system, you may love it or hate it, I think they're kinda neat, but expensive to get a new key, and the keys can fail.
All in all nice cars though, I'd buy one.

Gilly