If you do a cat back exhaust then no tune needed but if you go turbo back then yes you will need to tune. The Access Port is not required because there is an open source solution for your car. However, the AccessPort is very convenient and you can run the OTS maps that come with it until you decide to go with a pro-tune. You can even leave it completely stock and run the stage 1 tune for now but if you do go stage 2, I do recommend a pro-tune versus the OTS map that comes with it. If you go open source you will need a pro-tune no matter what. The key to a good tune is to find the right tuner. Find one that works only on or mostly on Subaru or has years of experience under his belt. Don't be afraid to ask your tuner questions but go in educated.
As for bang for the buck IMO, susppension is where you get most of your driving pleasure from. These cars could do with some improvement in handling. Sway bars and bushings, Steering rack bushings, shifter bushings and other mods. Get yourself familiar with the way the car drives and then you will be better prepared to handle the power later on.
Exhaust? I like the Invidia N1 street for your car because it has that deep rumble yet remains quiet enough not to draw to much attention. It is really good quality without being overly expensive but it is still pretty pricey. There are a ton of good exhaust systems out there. One thing to you could do is to see what others are using and maybe go to some meets and see what they sound like in person.