£5500 i'd say is too much no matter the condition.
haggle haggle. I'd pay about £4800 tops maybe less.
when test driving it check there is no knocking sounds from both the front and rear. If its the front it could be worn anti-roll bar bushes. Rear normally indicates worn suspension bushes.
check that it pulls cleanly with no hesitation. if if feels hesitant then mostly likely the MAF is buggered. cost about £80 on exchange from a dealer.
check the service history. haven the window clips been replaced? original ones are plastic and need to be replaced with metal ones. when was the cambelt last changed? should be done evey 60K.
let us know how you get onÂÂÂ

edit - see this thread
http://www.golfgtiforum.co.uk/index.php?topic=46541.0will give you a better idea of what you should knock the price down to