Personal opinion - If i had the time over, i would get one that already has the engine in it that i want. Dont pay for a stockish one and pay lots of money to upgrade it, as most said, be patient and wait for a Ceffie comes up that has already got a RB25 in it.
As for engine setup. Simple 3 choices -
Drift - SR20
Daily with decent power - RB25
Track - RB25
my 2c