Mid 90's C280s and E Class cars have a number of well documented problems that get pricey. On both cars:

Head gasket leaks (M104 engines)
Wiring harness failures (1993-1995 M104 engines)
Overvolt Protection Relay failure (M104 engines)
Mass airflow sensor failure
Harmonic balancer failure (M112 engines, early years)
Transmission problems with first year of 722.6 5 speed transmissions (98 model year I think)

On the C280:
lower ball joints wear out
lower control arm bushings wear out.

That's all I can think of from the top of my head. There is plenty of information on this site about these common issues. I have a 94 C280 and the car has had the head gasket, mass airflow sensor, the OVP, lower ball joints, LCA bushings PLUS the A/C condenser and the water pump. All this plus regular maintenance such as fluids, filters, brake parts, tires, etc. Since buying the car 6 years ago I've averaged about $2500 per year (Canadian, and that includes some body rust repairs) but I'm a little fastitdious when it comes to putting money into my car. Its been expensive, but I love it and wouldn't trade it in. Just be prepared to have it cost more than your SD.