There are significant differences between the RS, WRX and STi engine bays, you would have to swap out the entire transmission if you were to put in an STi engine. You would also have to swap out the front member as the RS one is slightly different and doesn't accomodate the additional exhaust routing.
Level of difficulty for the mechanical parts swap isn't high, the real difficulty is the ECU and other electrical parts changes required, including new wiring harnesses.
To save you any more questions, here is a link to the FAQ where all the swap info has already been shared:
http://www.automotiveforums.com/vbul...d.php?t=176338