The swap isn't that different from swapping into a GC. You can either do a motor-only swap and reuse your trans, diff, axles, etc, or swap the entire driveline over.
You'll need to keep the legacy steering column, so I'd recommend swapping to the WRX rack and doing the linkage mod
. You'll also need to use the Legacy radiator, as the WRX one won't fit.
Basically start by reading the stickys here and on rs25, then I'd be happy to answer any questions you have.