Hi there
I've just recently traded in my MK6 Fiesta 1.6TDCI Zetec Sport which I owned for about two years or so. I did have to replace the valve stem oil seals after noticing a strong smell of diesel in the car. That was quite an expensive repair bill but she was sitting on around 95K at the time.
From my experience as long as they're serviced regularly - oil change being essential due to the turbo then you shouldn't really expect any untoward bills apart from the usual consumables (tyres, brake pads, discs etc). Apart from the issue with the oil seals I never had a problem with mine. I also really struggled to get anything less than 60MPG regardless of my driving style. Granted they're not quick cars but then if you wanted a fast ish Fiesta then you should looking at the Sporting Trophies. . . . .