The front u-brace subframe thing is a bolt on/bolt off piece, so that shouldn't be a big deal to fix. (fun fact GC imprezas actually didn't have them, but the piece is for US crash safety, so while you can unbolt it and drive the car, the best thing to do is find a good used brace)
The back sub-frame is also removable but more work than the front as you've got all the suspension and diff and junk to move around, plus if you undo the rear links you'll need an alignment, so factor that into your budget.
The rusty subframe suck, but it's not a death sentence.
Get familiar with this site:
http://opposedforces.com/parts/
Note about tires. You do not want to run miss-matched sets of tires. You want all four tires to be matching in circumference to keep the center differential happy. People argue that it isn't a big deal, but I am very conservative with how I deal with things, often going the better safe than sorry route. If you have new tires up front and worn out back, over a long distance, the tires will have different numbers of full rotations. To make up for this, the center differential in your transmission is where the distance is made up. And after dealing with a pile of subaru transmissions, I wouldn't want to do anything to make them worse. My le-two cents.
Best of luck.
