RUST....its an issue on some focus's (not all)
Check all the bottoms of the doors and the tailgait bottom edge. Ford used some sh*te supplier of metal for a few years whilst building the focus (and i believe it effects the KA's of the same era) so watch out there.
Check the 2.0 Idles nicly. If it doesnt (and its a fairly common problem) then its most likely the ICV (idle control valve) they get past their best after a while and cause the car to hunt (idle tends to pick up and drop on its own) Its a quick and simple change. Should be fairly inexpensive too.
Other than thats its just ovious stuff. Being the 2.0 it will most likely have been thrashed (especially if its a 3dr) because most are owned by people who always wanted an ST170 or RS lol so check for oil leaks around the gearbox oil seals and headgasket.
Also check the clutch.. ovious i know but they are about £300-£400 fitted so its worth either finding a good one or factoring that off the price...
Other than that its ovious stuff. Good tyres.. No signs of head gasket failure... Good interior... tidy bodywork...
See if you can get hold of the options list from 1998-2001 with all available options and try and get one with everything!
The engines are fairly bulletproof as standard.