If you're looking to replace your suspension the best bang for your buck (depending on your intentions) are to purchase 4 new KYB GR2 struts. KYB sells products for ABS/NON-ABS vehicles.
If you're looking to lower your car substantially, coilovers are the way to go; BC, Tein, or Bilstein are all excellent companies.
If you just want the car to sit lower I suggest the Eibach kit or H&R's sport spring package. Eibach will take you down about 1" and the H&R's will take you down to approx. 2". Both are great springs that will certainly upgrade your stock suspension.
Other ways to upgrade your suspension are: new polyurethane bushings, sway bars, and bracing. I'd stay away from eBay for upgrade parts for your suspension. You really shouldn't mess around with these components. Your safety depends on it.
Good luck, PM me if you need any help, also check out RS25. That site has a lot of Impreza's like yours there, with a lot of information as well.