So, you want to put an EJ25 in the WRX? Ok, well, Im assuming you want to build a very strong turbo'd 2.5L motor first before you put it in the car. Leave the WRX alone for now, and save youself costs and hassle's. Build the 2.5 out of the car, take your time and do it right (you will also have a car to drive in until you 2.5 is done). Take it apart to save costs and send them to places you want to fo the work.
You will want to close (or partially close the deck), 8:1 compression, custom rods etc (heads are always good and turbo cams)...all the fun stuff, a raised redline might be a good idea as well, springs and retainers ar enesessary for this. But I personally wouldnt do the swap to the 2.5 until you are finished with it (if you are going to do internals on it, and other machining that is). Do everything on the engine you plan on doing before it goes in, trust me, you have to option to do so considering you have the WRX to drive around in all the time. Good Luck!