If it was a turbo one it would be cheaper. The NA's are higher in price (note i didnt say "worth more") due to the p plate laws. If those laws didnt exist you can bet it would be worth sh!t all.
Being a GTS4 at least its not that hard to get a crazy turbo motor in there after you're off your p plates.
Also, do you really think those k's are genuine?
All you can do is inspect it, get a skyline workshop to give you a report on it, then give the seller a reasonable offer and take it off his hands.