#BigSmoke :
That's an interesting pick, I myself also own a 4-door R34
(HR34 though instead of ER34, so it has RB20 instead of RB25)
GT-Rs are ultra-expensive indeed, so starting with a 4-door is a wise move
Also, I'd say you can get S13 or S14 as quick as.....now, since some of them are cheap, whether if it's an ex-drifter or not
In fact, I've bought cheaply, these prices are always sold with automatic transmission and I hate the automatic transmission.It's really expensive but it's a nice project car. I will do the necessary research to get s13.
I will do the necessary research to get s13. 
