I'm into the WRX STI for a few reasons actually.Honestly if you really want WRX, I would advise you to get Evo. The reason is reliability and availability of parts. It is cheaper to maintain or repair compared to WRX.
In terms of tuning cost, it's cheaper too. Even in stock, you can just raise the boost anytime without much worry. That's what I did to my Airtrek and I ran for years before I started the Evo-mods.
1. Boxer engine and its sound
2. Lower theft rate than Evo (I'm really very worried about car theft especially those who rip parts)
3. Interior trim (CT9A series Evo still looks too plain)
4. DCCD in WRX STI allows it to behave like a RWD car if I want it. Besides, STI is a RWD based 4WD
For mods, I'm not those who are chasing high hp. Anything around 300 plus is okay. Prefer minor mods and mods that enhances reliability of parts instead of more power.