Looking at a 99' C230 Kompressor
 

Need some feedback. Common problems?
The car I'm looking at has 142k KM. So 90k Miles? Is that a reasonable distance for a car 6 years old.

It is actually 7 model years old.
At an average of 20 k km/year, the mileage is right on.
My 2000 C 230 K has 131 k km on it ( bought it with 43 k km ), and I've had virtually no problems with it so far & don't expect any in the near future.
How the car has been maintained is much more important than the age or mileage.
If you're serious about the purchase, you could take it to a qualified M-B mechanic, pay him $ 100.00 & have it checked out really well. ;)