Prices are doing silly things at the moment, kind of makes me wish I had held on to mine. Some are saying £8-10k even for a non widetrack STi, so long as it has low mileage and is in good condition.
Main things I'd look for are the knocking rear suspension (can't avoid it really) - bad synchros in 4th, 5th & 6th gear, and then general things that you'd look for on any car.
Also you want to check for modifications, if they have been catered for with a remap and by who, what fuel it has been run on etc.
Simple cars really.