Speak to abbey and see what they have to say about it. As the advert mentions them.
you can get GTR33's for 15 grand but beware there are some dogs out there. get someone who knows skylins to check it out. check for false claims as well. seat belt tags should match the date of the car.. roughly. vin codes should be original.
make sure the vents on the rear diff are there (if they aren't then it aint a Vspec)
it is common to call a car a vspec to get it to sell quicker but they are not all genuine.
Thanks for the replies but a shame he needs rid of it quick as I can't buy a car until after christmas and I was just doing a bit of window shopping having seen a comment along the lines of you can't get a r33 for less than twenty grand.