The 1st rule of building a rally car as a novice:
Do not build your own rally car, buy a used one.
You'll spend $500 on the car, then at least $10,000 and a year making it into a rally car, and it still won't be competative, even with Petter Solberg behind the wheel.
Here's a good writeup on what you're getting yourself into:
http://linaracing.com/howmuch.cfm
For another reference, MattR and I were looking at picking up a *free* '83 GL Hatch for rally
cross and decided it would be too expensive. You're looking at buying a '93 Impreza, converting the drivetrain, and full blown rallying it. Remember, a rally legal cage is about $6000 for a basic setup... not to mention the $2000 suspension (the minimum you can spend to get something that will last one stage), the seats, nav equipment, fire gear, wheels and tires, truck and trailer, etc. All that's stuff you *have* to have just to be able to be legal and finish the rally.
I hope you've got buckets of spare cash lying around... converting the car to FWD is gonna be the cheap part of the project.
/reality check
Okay, now that that's said, I believe all the way back to the 80's Subaru's flat motors have all been 4-valves per cylinder. I'm not sure what the rules are for the rally series, but you might be able to simply remove or weld the center diff and take off the rear driveshaft/axles/diff and drive it FWD.