If you are really just looking for sound with the intake then just get a Perrin or K&N drop in filter. It was a noticable change in tone for my 2010 WRX.
As far as the BOV. Just do a few searches. You'll see that you are just wasting your money in that department. You can hear the stock BOV with a few modifications. I can hear mine with just the Perrin drop in filter and no other modifications.
As far as exhaust goes. If you want gains then you'll want to go turbo back exhaust (axle back is just going to give you a different tone). A downpipe is always a great first modification for a WRX. The stock downpipe is very restrictive. Cobb, Perrin, Invidia are all very reputable DP's. Exhaust you have a bunch of options (Cobb, Perrin, SPT, TurboXS, etc). This is where your ear will do the decision making...depends if you want quiet/loud.
Engine management. This is the most important piece. Also, a tuner. Cobb Accessport has any and everything you need. There are mulitple routes you can go with EM. It's up to your wallet and what you plan on doing longterm. If you can't say no to the itch...and you continuously modify your WRX. Then, do it right the first time and get an Accessport.
Your WRX responds very well to simple turbo back exhaust modifications. As far as what else to do. I could go on and on. It all depends what your goals are with the car. Goodluck.