I recently spent some time looking for a W202 for my sister. I viewed 5 facelift W202s .. 4 of them had rust on all 4 wheel arches. Having said that, mine (when I had a W202, also facelift) were clear.
MB give a 30 year paintwork warranty against rust as long as the car hasn't been previously painted by a non MB authorised repairer AND that it has FULL Mercedes Benz service history, so it would be worth taking it to your local main dealership to let them have a look
Dont think ive ever mentioned this, but anyone who has had rust on their w202 should remove their bumpers to check the areas which are normally hidden, id bet a pint* that there is more lurking!
Mine had a new tailgate at 4yrs old, due to corrosion, last year the drivers door was repaired due to corrosion now noticed front lower part of rear wheelarches are corroding also any stone chips are going rusty, The build quality and finish on these cars is dreadfull, mercedes needs to take some lessons from peugeot (cant believe I just said that) But my old 406 was much better built but not as good to drive.