To be honest, pretty much look for what you would for on any car, the petrols are pretty good if they have been looked after. as for rust, the 01-06 mk3 mondeo's don't really rust, completely different to the mk1/mk2, only place the mk3 did rust was inside on the bottom of the doors, but that was only on the very early cars, ford were made aware of the problem pretty quickly, and it was resolved on the mid/late model run cars, and they offered to do the early ones free of charge if the cars were relatively new. I had an 02 plate mk3 Zetec-S V6 for a couple of years, was a very good car, but it was only 4 years old when I bought it, so was pretty new at the time.