We bought a '00 C230SE new and 51k miles later we still love it. The only problem we had was a mass air sensor went bad around the 30k mile mark. Nothing since, great performance, gas mileage, nice ride. Love it!!
I have been threatening to change the supercharger oil when I get a few minutes though. The oil is prohibitively expensive but I figure once every 50k miles can't hurt. That's not part of the recommended schedule but I've been wanting to do it anyway for peace of mind. Keep the tranny fluid changed, as recommended earlier in this thread, along with the oil and drive it like you stole it, lots of fun!!
So far I'm on my third set of tires, (the second set was junk), second set of brake pads and rotors (went with Greenstuff on the front and ATE on the rears). Other than those few items, which are expendables anyhow, it's been cheap to own.
My only complaint is the engine sounds truckish, not exactly the model of smoothness. If you can get over that you'll love it.