a lot of great points and tips on the MK4/5 & 6 Fiesta's in this thread
I owned an 02 plate MK4/5 Fiesta for just over a year and it was a great car, very economical on fuel (37mpg on my comute to work which was mixed driving and around 42mpg on the motorway). Come MOT time for me it only failed on small things like suspension bushes (very common failure as they are poorly designed), CV & ball joints.
I can only echo who everyone else has already mentioned on here, the biggest gremlin on the MK4/5 models nowadays is rust - rear wheel arches, underneath doors, door sills, petrol filler cap & boot lids. It's really hard to spot one of these Fiesta's around now which haven't got any rust of them, whenever I see them about - they always have rust somewhere.
If you can afford to, I would highly recommend going for a MK6 instead. They are much more refined and better all around to drive compared to the MK4/5's. My mum has owned her 53 plate one since it was a year old and it's been a brilliant car and she has put 65K miles on it. The only thing mechanically that has gone wrong with it so far is one of the front springs that snapped last year. As far as I know as well, it's only failed 1 of it's previous MOT's which was on front tyres & brake pads.
As far as I am aware, the MK6 1.25/1.4 & 1.6 were ported from the MK5's and are exactly the same, except that on the MK6's they are labelled as "Duratec" and they have slightly newer other minor engine components.
Liam