Could very well end up costing you more than £8K easily if you were to need upgrade clutch/flywheel, better brakes (if you're still on all OEM), boost/oil pressure gauge/s etc as well or if any other unforeseen problems occurred like for example if you need new cam covers/gaskets + fitting if they were found to be leaking oil.
Bit strange to say though that you might as well buy an Evo/Scoobie/R33GTR if it's going to cost more than £8K though as they're all totally different to a RWD Zed with them all being 4WD turbo'd cars.
I mean if all you want is a faster car then yes you're better off just selling the Zed but if you want a faster Zed then you're going to have to commit to it.
My supercharged Zed ended up costing me about £1500 more than I had budgeted for but it was worth it in the end for the way the car drove with the power it had.
Did you see this STS kit that another member has & was asking what it's worth?
Link: http://www.350z-uk.c...-sts-turbo-kit/
Maybe he'd sell it to you if it's what you're looking at. Just an idea.