The amount of money you'd spend would be more if you were to source everything you need, than if you sold your RWD and purchased a Gts-4.
The other factor is that finding everything you need. You'd have more luck finding an honest cop than finding all of your parts.
The engine/sump/diff combo, the ECU, engine/dash/rear wiring harnesses, rear reservoir, rear diff, accelerometer, etc....
Good luck finding it all.
Really man, just sell your current beast if you really, really want one.
That's the best advice I can give.