I have heard of a few W202 C230K owners having to replace the Eaton M62 Supercharger at lower mileage, but other than that, the driveline seems pretty bulletproof. The M111 engine in aspro or ficed induction seems to be immune to some classic MB inline engine difficulties, including the headgasket problems of the M104. (Touching wood!)
The engine is very well designed with dual-row timing chains. The supercharged engine has beefed up internals including oil-sprayed pistons. This is to handle the additional loading of the supercharger system.
The HVAC system on the 1999- cars is different, in that it is "dual zone" but not an automatic system as found in the 1994-1998 cars. So far I have yet to hear any complaints about HVAC reliability.
Front lower ball joints are problematic on a few late model MB's, including the W202. My own C230 is on it's second set. Worse roads seem to mean more frequent replacement. One (not all) symptom seems to be a squeak when the front end is "bounced."
There is real conflicting info on changing the tranny fluid. The dealer tells me that it's fill for life. The Zone Service Rep says change it every 100,000 kilometers (60K-miles or so). I had mine changed at 90,000 kilometers hoping to err on the safe side. I had the dealer do it, as the labour charge is reasonable, and the tranny a little tougher than others. (the fluid chould be at 80C to set the level properly)
The other potential driveline weak spot on W202's is final drive seals. The axle seals will sweat on some cars. Mine did this some time ago, and since it was under warranty, they replaced all three seals. Two years later, and not a sweat yet. (Touching wood)
Rear sway bar bushings are also a sort-of-common repair, and they are noisy when the rear is "bounced."
Check for a good service history! Have a PPI performed by a mechanic that knows these cars.
1998 C230 "Black Betty"