Rust is a big concern. Indeed, many ST220s were repaired and repainted under warranty due to bodywork corrosion.
Pre-2005 Mondeos are especially susceptible, especially around the door bottoms, which are poorly flanged and trap moisture, leading to severe rot. The rear wheelarches may also suffer, so check them carefully, along with the bonnet edges, inner wings, tailgate, beneath the fuel cap, around the windscreen and back lights.
Saloons and hatchbacks have trouble with their rear bumpers sagging after a while, which can be cheaply rectified with new foam supports to replace those that are either missing or broken.
The choice of bodystyle is down to preference, and these days makes little difference to the purchase price. The hatchback is practical and plentiful, the saloon offers little more than a mildly stiffer shell, while the estate is enormous – if a little slower and featuring a slightly inferior rear suspension setup.
Colour can have a strong effect on desirability, while some folk prefer the fairground chrome trim and back lights fitted to 2005 models. Xenon headlamps are a worthwhile optional extra to look out for, as is the rare electric sunroof.
Beware of white ST220s, which were generally purchased for police use, although other colours were available ( especially silver). Such cars had hard lives, so look out for damaged and scratched bodywork, potentially having filler in the roof.
Most importantly, buy your Mondeo based on condition rather than looking for the lowest price. It will probably save you a packet in the long run.