Have a look through the ads on
MCN
A quick glance says around £2750 at a dealer should get you something similar, private under £2.5k I'd suggest.
When buying anything the most important aspect is the paperwork, doesn't matter how nice the bike is if the paperwork isn't legit. HPI to cover yourself against outstanding loans, stolen, crashed/write-off etc.
Consider carefully any imminent costs, tyres/chain/battery/MOT etc.
Trust your instincts, if the seller seems iffy then walk away. Only view at the registered keeper's address, check watermark in V5C document (says DVL all over it when held up to the light), MOT can be checked on-line now, easy things to check. (2006 bike won't have MOT yet)