Genuine GTR yes, As for as the quality of it, I don't think anyone here can tell you to buy it.
Has some positives,
low kms
service history
looks clean
plenum not flaking
couple nice modifications
As said above get someone with great knowledge of these cars to have a look.
And Listen to the car both on a cold start and warm start.
And also check everything as (OO)SKYLINE(OO) said, and we mean everything.
This could also worry the seller and you may get some body language to show he is nervous and worried about you finding something.