That is realistic yes.
I wouldn't say that a lot of the private sales look suspect though, most people who buy and own a Golf GTI tend to look after them to a certain degree. Some more than others but in general if you get someone who knows what they are looking at and HPI check the car you can save quite a bit buying private.
As a guide, I paid £7k for my GTI 3 months ago. It had the 18" Monzas, ED30 front splitter, ED30 skirts, R32 rear and a few other nice little mods. 72k miles on the clock with recent cambelt change and detailed service history. No problems so far! The condition wasn't 100% great, a few kerb marks (I have replaced the wheels) but overall a good buy for the cash. £8k should get what you are looking for!