1) Brand new if you can. Used sports cars are a headache nobody wants if they can avoid it. 15 grand down, sounds like you can afford to avoid it.
2) any aftermarket part will lower the value of the car because it increases the odds of something being broken during the install process, increased strain on surrounding parts, and just the general rate of abuse from the driver
3) You want to read a lot more about car modification before jumping in. You're safe with a brand new WRX from the show room that you leave stock. If you start changing things or even worse start dealing with things somebody else has changed, you need to have a much better understanding. This site is a good resource so do some reading, ask some questions. It's a fun hobby but it's one you can waste a lot of money in if you don't know what you're doing and veer too far from best practices. If the thought of 15 grand going up in flames in a "failed experiment" with your car, then you need to be careful and remember your car first and foremost needs to be daily reliable transportation.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD