awesome build man! great to see more people making subaru drift builds! I'm sure you will be very pleasently suprised once you get the car sideways at how amazing they are for drifting!
Be sure to check out Derrick
builds if you haven't done so already. Tons of helpful stuff in each of them.
Biggest advice I can give you is put considerable time into modifying your steering. If you have GC non-abs knuckles, they are perfect for drilling in order to get more angle, and drill significantly in as it makes a world of difference. On the other hand look into where the front tire will hit as on my GDB it hits the frame rail right where the cross member bolts and i had to knotch it significantly.
I'm assuming the impreza L was FWD, and if so that rack travels further lock to lock than other suby racks and gives you roughly 10 degrees more steering angle.
In terms of the rear diff, r180 or r200 are both really fine. Just make sure you don't use the stock rear axles as they will snap in a second. Check out this thread
for a lot of good pics on getting the r200 in there!
Can't wait to see this thing sideways!