Hi,
I would have to disagree with the comment regarding Auto, as there is nothing inheritently wrong with a manual version of this car and having driven an Auto version I personnally would,nt have one and would always go for a manual. But this is a matter of personnal choice.
If its a w202 (up to 2000) makes sure there are no untoward clunks from the suspension, and that the steering wheel is straight and the tyres are wearing evenly on the front, ensure the car pulls cleanly and smoothly through the gears.
If he buys an Auto, make sure it has had the ATF and filter changed within the last 20-25,000 miles, if not budget for it.
Rust wise prefacelifts were IMHO better built (body wise), but due to age, look for rust in the common places around the front and rear arches, boot lid, etc, but generally these cars did,nt (or have,nt yet) suffer from structural rust, so if any rust it should be quite easily visible.
My C230K Sport I have had now for nearly 10 years and a 130,000 miles, and it has not let me down, until last week that is with a suspected gearbox failure, which I believe may have been caused by incorrect maintenance (which I,m going to discuss with Olly when he repairs it for me). It has only cost me normal service patrts and wear and tear items such as tyres and brakes essentially, and engine wise just a water pump and a set of plug leads.
Best car I have ever owned, and I have owned a few.