I would just buy a 2.5 short block (I got mine w/14K miles for $1100) and use your wrx head...no wiring changes needed. Everything will fit on the 2.5 block. You'll need a gasket kit including head gaskets, and all the fluids. I have no idea how much it would cost to have it swapped out; I did mine myself, but I would imagine you could do it for ~$2K if you bought the parts and had someone install it.

While I was at it I swapped the clutch, flywheel, crank pulley, motor mounts, oil pump, timing belt, water pump, and accessory belts because it's alot easier to do it when the motors out and I had almost 70K miles on it anyway. So, obviously any extra parts that you decide to change (some I would highly recommend depending on how many miles on the car) will cost you extra. My whole swap (parts above plus a few others) cost me $5K and I only paid $500 to someone else to have the head reconditioned, the block inspected/honed, etc. The rest was the cost of the parts.

It's very difficult to do it right and cheap at the same time whether you go with a 2.5 or a 2.0 IMHO.