M104 engines have a fairly high failure rate for the head gasket.
The pre 1996 models may have a wiring harness issue. Look for one that's replaced, and if not, carefully inspect it.
Otherwise, the C280 is very reliable car.
The M111 2.2 and 2.3 engines seem pretty bulletproof. I've got a few friends with tons of miles on 220/230 cars, and they have held up very well.
I get about 9.5L/100km (about 26USmpg) city driving and 6.7L/100km (36US mpg) highway in the C230. I think the 97+ C230's do a bit better on the highway thanks to the taller .83 fifth gear, absent in the -96 cars equipped with four speeds. But it's marginal.
My C230 has 104,000 kilometers, and still has yet to use a drop of oil. Not a drop.
1998 C230 "Black Betty"