The sum of my Mondeo Mk3 buying knowledge is thus, mostly gleaned from this forum:
Early ones can rust - check the bottoms of all the doors in particular, and the rear wings. Rust is overed by the anti-corrosion warranty but only if the inspections have been done when its been serviced by Ford (many garages will not bother to complete this section of the service book). Models newer than 2004 should not really suffer.
The TDCI Diesel engines are decent enough and feel quicker than the 1.8 and 2.0 petrols thanks to the torquey delivery, but sadly they suffer reliability issues relating to injectors and dual mass flywheels and thus can leave you with four figure sums to replace these if you get problems.
The 2.0 petrol Duratec engines can suffer from failure of the auxilliary belt tensioner assembly, its not megabucks to replace if it goes although I believe a revised part was issued that cures the problem.
If its performance you're after, buy the ST220, fit exhaust and delight in the sound, whilst murmuring nervously about the relatively poor brakes for a car of this performance, for it it shares the same setup as any other Mk3 Mondeo You'll find then find you're going a lot faster than it feels (the power delivery is very smooth and it picks up speed quickly but doesn't necessarily feel it!).