Can't comment on MPG, as have 2wd sapph, but reliability I can.
They can be as reliable as the next car if treated right, though some parts are getting hard to get hold of, from fords anyway, but most stuff pattern ok.
Must be the easiest performance car to work on DIY, even the timing belt is dead easy.
Use a fully synth oil and take changes to 12,000 without problems.
Very cheap to maintain too.
I wouldn't have trusted mine when I bought it, as had been neglected, but was what I was after, so done most jobs now, just been stood 4 weeks with alarm/tracker on(charged once) started up on the button, never missed a beat.
The ecu's are a bit antiquated these days, but fine if you don't do loads of miles, if you do a more modern one would benefit the car big time.
I would personally go for a stage 1, minimum, as they are a bit sluggish as std compared with modern tdi's.
tabetha