if uve got the do$h then do the whole zaust system , extractors back its worth it if u can afford it, if not a cat back is fine.
Dont go higher than 2.5 inch and dont go lower than 2.1/4 inch not worth it.
To avoid attention from the boys in blue dont get a fat, angled cannon, a simple twin tip will suffice, plus it looks stock.
Heat wrapping or heat coat the headers either or both, depends how fussy u are.
You will be able too feel a difference from your new system, makes the car feel alot more responsive
if u are in the sydney area google 'best mufflers', 'liverpool exhaust' and 'tuffy mufflers' ring around and get some quotes
and lastly, enjoy your ride