i plan on using the same transmission and the same dash. my cross member is already notched so i was wondering if i could still use that. but how hard is the wire merge to do?Read that list in the link that Joshua posted. That is what I went by when I did mine. Also, are you keeping the stock transmission? If so, you'll want to upgrade your clutch as well.
If you are planning on keeping your stock dash then you need to merge both your stock and the WRX bulkhead harnesses. If you are swapping a WRX dash then you can forgo the wire merge and will need an entire WRX bulkhead, engine harness and fuel pump controller. You will also need to swap out your front crossmember with one from either a WRX wagon or earlier legacygt . WRX sedan crossmember you'll have to use washers since the track for the control arms are a little it wider.