Never drove one, but I did my research BITD when a friend wanted to import one from the UK.
If it's the R34 GT-R model you're after, it'll be difficult to find a good one that hasn't been tuned. If it's tuned, you're taking a big gamble. Not only is it hard to tell if it's been tuned professionally, but it usually indicates that the car has had a rough life already.
A GT-T (2WD, turbo) is also tuned most of the time, and usually by people who couldn't afford a GT-R to begin with. So that's done on a budget.
If you really want to get one within the UK, your best bets are the less popular models. The non-turbo GT or GT-FOUR variants.
IMO it's better to look for a good one in Japan. The majority of those aren't tuned, and they're usually sold dirt cheap.
In Japan, when a car hits 100.000km (62-ish thousand miles), by law the car either needs to be scrapped or the engine replaced. Many Japanese therefore sell their cars when they're close to that point, so it should be easy to pick up a good, unmolested cheap one.
Quite a few GT-R owners don't drive them too often, so there still are plenty that are still on their original engine and about to hit the export market.
Sure, you'll need to import it to the UK and probably have to change the speedo to make it roadlegal, but in the UK that seems rather easy. A lot easier than in most of Europe, where it needs new headlights as well and they tend to be picky about having the steering wheel on the other side of the car.
Anyhow, there are companies that specialize in hunting down, acquiring and importing Japanese cars. Google around a bit.