God the "mondeos are crap" brigade are annoying - they seem to be missing the point completely.
Anyway, potential problems on the ST TDCi are DMF and injectors, turbos are fine generally (well, as fine as any other turbo diesel - that is to say a potential trouble spot but nothing serious)
The DMFs were uprated in 2005 when the 2.2 TDCi was launched - so in theory any ST TDCi, 2.0s post 2005 or earlier cars that have had the DMF replaced should be much more reliable. Again, they're still potential trouble spots but unless the car has had a life of hard launches, clutch riding and rough gear changes it wouldnt give me cause for concern.
Injectors, like any turbo diesel can be problematic but really only at higher mileages (upwards of 100k). Again, if the car has been looked after it wouldnt bother me unduly. Running them nearly empty on fuel, driving them (pointlessly) to the redline or fitting "tuning" boxes are the sort of thing that kills them. If one fails, they're tricky to diagnose and some garages will tell you to replace/recon all 4 - which is where the reputation for 4 figure bills came in. Specialists will be able to bench test them though, so unless you're unlucky enough to have more than one fail at a time, its a sub £300 bill for diagnosis and reconditioning an injector.
Other than subframe bushes on the estates, there really arent any other issues - would be a step down from your VXR but not a bad choice as they'll do 50mpg
