Can't give you much specific advice about Fiesta's, I was supposed to buy one as my first car but bought a Mercedes instead.
Check underneath, see what the overall condition is like - whether its rotten to bits or not, front suspension arms etc.
Check tyres for badly uneven wear patterns, could indicate something more than just tracking being out.
If you can drive it, drive it in circles at slow speeds on full lock - listen for knocking noises, indicates worn/damaged steering joints.
Listen to the engine starting up from cold - if its warm when you arrive, ask yourself why? I had a quick test drive of a 1.25 Fiesta, and although the car was in lovely shape the engine was rattly as a bag of spanners, even when heated up!
Walked away from that one!