The cars are essentially the same from '02-07. There are minor changes, so it's safer to source parts from a WRX of the same generation (go by the type of headlight). In terms of suspension, you want the WRX suspension parts as the hubs, axles and overall suspension, and the driveline layout is identical to the WRX (but not the STI). Power-wise, its going to be hard. The EJ25 in the base model Impreza is a single-cam motor and, of course, NA. While the physical location of the motor and trans are the same, much-if not all-of the intake plumbing is different. The only thing that will bolt on is an after-cat exhaust, and that might be overkill for a 2.5-liter, NA motor tuned from the factory for torque. If it's affordable power you want, I would look into a motor swap from a WRX.
Jay Chen