An RB25 will fit, you cant use the GTS-T gearbox though, remember Stageas come with an RB25 and awd. Cheers blind elk, didnt know the sump patterns are different!
You would be best off buying a manual Stagea front cut with gearbox, that might be harder to find but you will/should get the gearbox/pedals/brakes etc etc.
To be honest you might as well save your money now and buy a turbo car later on...