RB Engines are tough, Here's a 25t conversion.
Few points to note...
1) Buy the engine with the gearbox and factory ECU
2) Better with the speedo, (differnt ratio. the cost of calibrating is nearly the cost of a 25t speedo
3) U can't use the factory boost gauages in the 25t speedo. (big job)
4) It will be great if u can get the factory intercooler piping, otherwise, the piping can cost u a lot.
5) 25t Air Flow with pink sticker, 25 is green one. (90%)
Engine from Parts Import => $3.0k (Engine start Garanetee & Parts support)
Gearbox and drive shaft from NSC Parts => $1.1k
GTR Clutch and Fly wheel set from ST Hi-Tec => $700.

Price subjected to some shit lucks.... u will say some parts are $expensive$ but plz do compare the condition.