Welcome to the board!
The 17" vs 18" debates rages on! I'm actually one of the people that put 18's on his Impreza on stock suspension. I got tired of it pretty quickly. With the softer stock spring/struts, the low profile tires you have to run on 18's end up being too stiff. They cause the tires to "chatter" over bad roads, losing traction and triggering the ABS if you happen to hit some rough road while braking. I eventually sold the 18's and went to some 17's. Not only is the rubber much cheaper, the handling is better as well. In fact, now that I've converted my WRX to my race car, with coilovers and heavy swaybars, I'm still sticking with the 17's... they really are the optimal size for performance on Imprezas.
Also, if you're interested in decent wheels on a budget, a lot of people order Rota Wheels (they make decent replicas of popular wheels) from
www.subydude.com. For the sway bars, I'd suggest Whiteline as they're cheaper than the competition and have a great product. Other good swaybars are made by Perrin, and Cusco, but they're a bit more expensive.
I'd do what other's have mentioned in this order:
1) 17"x7.5" wheels & 225/45/17 tires
2) 20-24mm adjustable rear swaybar (or just a 22mm fixed bar)
2a) Rear swaybar endlinks
2b) Rear swaybar mounting bracket
3) Rear strut tower brace
If your dealer is really strict, they might call you on the swaybar if you come in for warrenty work on something like the struts. But for the most part you should be able to do the items above w/o warrenty concerns.