Aren't Litchfields (GTR specialists) doing a warranty that includes covering any mods they do to the car? Sure it was around £1k/year which seemed reasonable.
I don't think te running costs are that bad, I certainly wouldn't buy a GTR, new or second hand, on finance and then try to run it. It's definitely next on my list but only once I've got enough to buy it outright otherwise you just know it'll go tits up!