Check thE seat bases ain't cracked. Operate the hight adjuster and wiggle the seat, they are prone to breaking and one corner will drop when sat in.
Alternators commonly fail and are an expensive fix.
Drive shafts are a weak point.
Models about that age have weak clutches so expect it to slip if you map it, but you could be lucky an have a strong clutch.
Oil diaphragms fail. Best repaired by using the Volvo part instead of ford.
Boost and oil gauges can play up usually due to a duff signal from maf/map sensors. Disconnect the batt for half hour would usually sort it if not new sensors.
Rear gearbox mounts are a bit shit, updated one is about £65.
It may have a space saver tyre, if not a compresor and tyre weld should be in the right hand corner of the boot.
Abs module can fail, £1400
Side skirts can come away from the body so check they are fitted correctly, covers for the headlight washers also come loose.
Remember the st3 only has 4 seats as well.