When I came to get my Zed 18months ago (maybe nearly 2yrs now), the GT cars were demanding about £1500 more than non GT. Even then, no one was touching non-GTs. I think its a hard call, depending on how long you keep the car, whether you want syncho-rev-match, which lets face it, is what the pub convos will be about when you say you have one
I'm sorely tempted to get one (when I get my new contract at work), but my head says to wait a year and see what residuals are like. In the current market, I can see you getting a 1yr old 370z GT for about £20k easily if not less!
Also, I'd go with the 19" wheels. Dont think the 18s fill it enough without larger profile tyres ( ), and the 20s from the GTR just look too big, even if you used lower profile tyres.
As pointed out in the GTR wheels thread, look at the resemblance to the old Zed, really does follow the old design lines