No really a fan of the GTO's, the 300zx TT is a nice bruiser but your money would be safer with a Supra as the 300zx's will have been owned by people who can't afford to run them, so finding a nice one will be hard.
£5K is considered the starting price for twin turbo supra territory and you should be able to pick something thats clean and reliable up for around £5-5.5K. However forget about single turb's cars for £5k, those tend to fetch twice that.
Also take a look on the classifieds on this website, you may see that suits you. I searched all the adverts/sites (autotrader, piston heads and after 1 year of hunting got my car from a fellow member on this website).