I'm confused, you want to buy a normal impreza, and upgrade it to 400-500(whp?)? in that case your going to have to upgrade the brakes (maybe the lines as well? I'm no mechanic so i'm not sure), get a new transmission/clutch etc to handle all that power, upgrade the suspension, get an entirely new engine, a tune, etc..
So if you plan on doing a lot to the car anyway, I'd say go with an STI to begin with, less headaches down the road, stock parts handle increased power more reliably than impreza and wrx. Also, I'm sure others will chime in, 400-500whp is a ridiculous amount of power, and not a great idea for a daily driver.