Hi John, welcome to the forum. You should be able to find out most things you need to know on these pages.
The thing is, any car is only worth what your prepared to pay for it. I have seen Z’s going for under £4k which to me is sacrilege (especially when you consider the rear screen is @ £3k on its own to replace !!!).
The condition is everything, most Z’s have been cherished by their owners, but not all, so it’s important to make sure that it’s been mechanically well maintained, and of course the usual checks about accident damage etc.
Have a read through various threads on this forum & you‘ll find a few of the more obvious things to look out for. As it’s a Mk II you don’t need to be quite as concerned as the Mk I owners for things like timing chains, but things like sills & services still need to be ok.
Mine is a 63 plate GT200 with full service history, 1 previous owner, and I would like to think it’s in pretty good condition, although mine has done 48k now. I recently was offered £6.5k trade, so I think given the mileage, condition & service history, plus the fact that as its only had 1 owner means it wasn’t a Friday afternoon special, so £8k is probably fair for a retail price.
To be honest, if you paid twice that much, you still wouldn’t be disappointed in you new Z.
Enjoy