Just the usual stuff. When looking at any car. Make sure it has a full service history. For the age the car it shouldn't have any issues, bushes and bearings etc would be fine unless extremely high mileage. Same with suspension gearbox etc.
I don't have any issues with my 08 and haven't had to do anything except regular maintainence, I have replaced the tyres as they were worn when I got it. And the wiper blades. That's it.
Pretty sure Salsa and Jay will pop along and share the ownership experiences.
Might be worth posting the ad here so we can have a gander.
There are basicly 2 versions of the Hatch STI.
Type UK. The "standard STI" 290 bhp 17" wheels.
There was an option for a Prodrive performance pack which changed the exhaust back box and centre section plus a remap to 330bhp. A few of these came with 5 spoke 18" alloys
330s: same as the Type UK. 330 bhp. 18" alloys, Recaro seats, factory Sat Nav, electronic push button start.
There prob a few things I've missed off though, I'm sure someone will point out my errors :)