That Steel Grey GTI sounds good but i'd want to know a bit more info:
* FVWSH?
* Genuine non-grey import?
* Exact date of reg because if it is anything upto July 07 then it needs the cambelt and water pump change
* 17" or 18" alloys - if of personal interest
otherwise it looks a good example with low mileage
However you always have to go and have a look, take your time, don't be rushed, check everything external and interior. Then if all is ticking boxes start it from cold, take a test drive, bring it back and just let it sit idle and check that it isn't under revving or over revving or that the car isn't unduly shaking when idle. Check all the electrics - windows, lights, windscreen wipers, wing mirrors, etc. Then switch it off, wait a minute or two and then re-start it. (at all times tell him what you are going to do and if needs be what you are looking out for but the seller will know all this anyhow)
Then try and knock him down to 11k or better if possible. Good luck