do a search this has been covered, but a refresher is always nice.

is it worth it? probably not.

not only do you need an engine + drivetrain (gearbox , diffs?), you then will need to tune it, at least a new bonnet for TMIC or stretch for a FMIC.

on top of that you miss out on the wrx goodies like the momo stering wheels, rims possibly leather seats.

if you could find a mechanically sound write off it may be finacinally viable to tranfer most of the bits.

but if your paying trade price or more, then adding labour. go buy a stock wrx.