Ive got one for sale http://www.wrxatlanta.com/forums/showthread.php?t=10801 Let me know if you are interested or have any questions
Any catback system that is 2.5" or larger will be fine. You just have to think about the amount of noise you want, because that is all a catback is really good for (maybe 5hp, unless its part of a turboback).
Thats what I'm rockin with a Helix catted DP. It sounds great but kinda drony and highway speeds. Plus its really loud, gotta be a 9 on the Milo scale!