Add what the first reply advised and run it as is in stock class. That's your best bet.
Here's a budget builder and a buddy did for under 1500
If you want more in depth, here is my BD build for m4
I would highly suggest looking up class rules and building to a set of rules for the class you want to race and remember the more you modify, the more difficult of a class you'll run. A motor swap will pitch you straight into modified against log booked rally cars and fully prepped rally x cars. I believe rear diff would put you in prep class. More info on this at dirtyimpreza.