I've got a fully tuned up '03 Zed on the game with a supercharger but thought as Stu hadn't charged his on the game I'd try to do it the same.
Anyway if you've no money in the game how do you expect to buy the supercharger? GT6 doesn't do "layaway".
So the specs you're aiming for are:
Fairlady Z version S (Z33) '07 ~ fully NA tuned, no custom gearbox or suspension, no NOS or FI, custom LSD, stage 3 weight reduction with racing soft rubber tyres (oh & BGW if you want).