Having re-built 2 MG's, a VW Camper, 2 Lotus Cortinas, one replica Ferrari and 25% through another I can tell you that you WILL spend £10000 on top of the kit (£5000) and the donor (£5000 +) so in the case of a Z8 replica on a decent Z4 3.0 about £20000. If you could find the kit anymore that is.
Having said that a really well-done Z8 replica would fetch £35-40k and will be as rare as rocking horse poop in the future.
I stress the "really well done" part on a good donor otherwise, crap in-crap out.
These are not kit cars, rather a re-body of a good car. Early kit cars where the whole car was a bitza made in a shed from old cortina and herald bits, were not worth much and some were very badly done (but the owners were pleased as punch), what is in discussion above is a very different proposition and well worth doing.
My 250swb Spyder Replica is no expense spared, super-detailed and high-quality build (sell for about £30k in this quality) and the 250 California Replica I am building now will be the same (sell for £60-80k which shocks me but they do sell). You can drive these all day long every day as opposed to the real ones a $20 million that sit in a garage all their lives. Mind you Chris Evans did lend his to James May for the day for a drive and you see them racing around Goodwood 6" (or closer) to another £10 million+ car.
If, like me, you enjoy the build process and creating something yourself, then go for it. If you are perfectly happy with your Z3/4 then stick with it, If you are happy to modify or restore your Z3/4 to concourse condition then go for it too, if you are happy with an untidy runabout for the wind in your hair then go far that too.
I am happy whichever way (but happier with the replica!).
Each to their own and criticism of someone's passion serves no purpose other than to make the critic look foolish.
My 2-cents worth! Let battle commence