The 5 speed electronically controlled transmission had some early controller issues that may have been worked out by 1999 - MB had to retrofit controllers, etc, into a few 1998 models with that trans. Check it for leaks as well, as there is a fitting on the trans that is prone to leakage, and was updated by MB. If buying from a dealer, I would consider requiring a fluid/filter change in the trans as a condition of sale. MB says the trans is 'sealed for 100k miles, but the fluid is expensive, and the change process is complicated, so might as well have them do it on their nickle.
Also, some techs feel that the compressor isn't up to the same quality as the rest of the engine, and was a weak point, but a number of members here have gone fairly high mileage without problems from the compressor. Try a search for different combinations of the key words w202 (chassis/model number), C230, transmission, compressor, blower, supercharger - you should find a number of threads.
I would check out the suspension (especially front) and brakes pretty carefully, especially the front suspension, as several members talked about early wear in these areas, but we haven't seen any problems on ours in front. We had a problem with the rear differential mount and sway bar assembly, however, that was replaced under warranty. (Not unique to MB - several of our friends with Toyotas and Subarus had the same rear sway bar problems).
Finally, I think the climate control in 1999 went to the seperate knobs to control each side of the car individually. If it still used the 'radio sized' push button LCD display unit in 1998, it is worth having this tested carefully for all functions, as it is expensive if/when it fails.
Otherwise, we love the handling and ride of our 1998 C230, and the 35 mpg highway is hard to beat. It was a bit soft for our tastes, so we installed Koni sport shocks and the 16" sport wheels from the C280 sport, and it handles very nicely at this point.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E