300kw at the wheels is pretty decent power. You are going to need to change turbo's, ecu, tune, injectors, fuel pump (depening on what it comes with), clutch (depening on what it comes with), exhaust (depening on what it comes with), to put that power down you will also need good tyres/suspension (depening on what it comes with).
Its kind of a hard power level because if you want 200-250kw at all 4 or less you could do it without having to change as much, but beyond that you are really going to need turbos and then all the supporting mods they require. Also if you plan on driving it daily and you hammer it, the stock internals probably wont last.
All I can say to you is that GTR's are NOT cheap, particularly if you have got a few mods, also if you buy a cheap example and mod it up you a really going to want a rebuild otherwise it will just blow up and with that and mods you will be looking at over $30k.
If you went for a 34 GT-T it would be cheaper and with the right turbo / ecu you could get that power but I don't know why anyone wants a car with 300kw at the rear wheels for the road... You wont be able to utilise the power.
If I were you, I would buy an s15 for about $22k (one that has decent wheels and tyres already and most have exhaust system). Then you just buy intercooler, power fc and get a decent tune and you will have just under 200rwkw in a car that weighs <1,300kg... It will be plenty fast and fun to drive and also be useable and for less than $30k.
thats my 2 cents anyway.