Ask Brad, aka CruiseLiner, he's spent like ~$20k and upwards on his RB26 RS4.
IF you want a RB26, DONT buy an RS4, buy a 260RS, they come with a RB26 and HEAPS of other stuff, and is a much better base for BIG power compared to a RS4.
So buy a 260RS ~$30k, add $7k to $10k for the kit and volia! It still does not make it a GTR wagon but, it will always be a Stagea