Hi Scooby Toon and welcome to TorqueCars. You have a WRX - is there not enought performance already for you? How much are you wanting to spend and what do you want? Silly power and no reliability or sensible power and good reliability.
Interior wise there is so much to do. First off I would get some nice sport seats installed and then look at a complete retrim of the dash. Have you got any idea of colours?