The biggest problems to worry about with a 1999 c280 (or any other used car) would be the previous owners. If you find one with reasonable mileage that has been cared for (with the records to prove it), you'll have yourself a great ride. Some common gotchas are AMM, crankshaft position sensor, fuel pump relay. There is a service campaign for the harmonic balancer:
http://www.peachparts.com/shopforum/showthread.php?t=119790&highlight=harmonic+service+campaign)
If it has the original battery, don't wait for it to fail(it won't be long), get a new one. You will have to re-code the radio and re-synchronize the windows and sunroof. Also, service the sunroof with the correct MB lube.
Good Luck, Dan