They are great GT cars, but they are also great B road cars also, especially with a few suspension mods such as coilovers and Eibach ARBs. There aren't massive power gains without going FI, but you can still get a good 10-15% with the correct breathing mods. I'm almost certain that a stock zed will take a stock 645 on most tracks in the UK. Those with long straights like Silverstone will be different because the BMW V8 will stretch its legs and can easily take a couple of seconds off the Zeds time on a big straight.
Buy the car, they're great right out of the box. Of course everything can be improved and no matter how good your car is, there will always be something better, but for 5-7k, there is no better all rounder.