The car looks nice mate but the fact that he's specifically put that it's only had one owner from new yet your HPI check has said differently would be really worrying to me.
Doesn't mention service history either.
I'd request some more info before you go and see it mate. Get him to send you a picture of the V5 to prove the one ownership for a start. Then get him to send some pics of the service book and ring some of the dealers to double check it's actually been there.
If he genuinely has been the only owner then I'd expect him to have all the paperwork in order and stuff.