Having owned our SLK since 99 and been active in the MB community, this is the first I've heard of leaking problems on these cars. That's not to say there aren't other problems with the car, but a leaking roof isn't one of them.
We just had to replace the supercharger (at a cost of around $3K), and the car only has 36K miles on it. We've had work done on the passenger door/window 6 times before they finally got it right. There's a known problem that plagues the brake lights on all SLKs before 2003, where one of them will intermittently stop working. MBNA hasn't found a fix as of yet, so we carry a print out showing the documented issue, in case police decide to pull us over.
Great stuff on a $50K+ car.
If you're going the SLK route, get a 2000+ model with the V6. Prices have come down considerably, and you should be able to find one in the same price range.
