Best bet is to check out Scoobynet.com
I would say that 5.5K for a Hawk STi seems a little cheap tbh. Check to see if it's on a register.
I wouldn't touch one with a turbo timer. Useless bit of kit that shows the owner has now clue what they're doing.
To the person who said a service every 3K @ £500?? I ran a heavily tuned Bug. Fresh oil / filter and sump washer every 5K, or track day. Silkolene Pro S race oil with Subaru filter and washer = total £55 and SO much easier to change than a VW.
I ran EBC yellow stuff pads and their uprated rotors. Great stopping power, and although a little dusty, lasted a good long while.
Fuel will be the killer - they drink it for a hobby.
