Lots of commentary recently about the W210 E320 in the last few days. Do a search and read it carefully. If your dad is not afraid of routine and non-routine maintenance work then the E320 is a great car, one of the best I think for what it delivers.
Read the first post in this forum about the balancer failures and do a search for other mech. problems too. This site has a very good search engine (no pun).
All cars have problems but if you are looking for the reliability of a Toyota or Honda the W210 is probably not that car. If you are looking for a great, consistent ride, plenty of power and an unearthly feel of a car then the Toyota or Honda is not your car and the E320 is.
Between the gas or diesel W210 - I lean toward the diesel because I own one and love it dearly. It is the ultimate cruising car that can chirp the tires too on initial acceleration. Good luck and do plenty of research.