First off when you're shooting for a specific whp # you really need to figure out which dyno you want to make those numbers on as they all read differently (and even the same one reads differently day to day). To the guy making 380 on a vf39 on e85 those numbers are HIGH, but I've seen it done on specific dynos. The numbers in my eyes are the worst way to judge a cars power, a 1/4mile trap speed will be a much better indicator of how much horsepower the car is actually making.
In reality if you are really looking to make 300-320whp then a simple 2.5L swap with a vf39 on e85 will get you there. Even at that power level don't expect the crappy gears on the earlier WRX transmission to last very long. If you're looking to make anymore power then that a six speed swap will be in your immediate future and cost you about 3-5 grand depending on where you source the parts and if you do the work or not.
If it's just a daily driver, in my opinion, anything more then 300-320whp is a complete waste on the streets. The bigger turbo will have great lag and you'll be far exceeding the speed limits and endangering lives in no time. If this was strictly a track car then you're better off saving some money and getting an STI as it's a much better platform to start a build on.