Don't think you can beat a Magnex cat-back TBH.
Pipers are quite good as well.
4" looks fine, ST ONE has a 4" slash cut IIRC, looks ace!
(when it's out of his boot

)
I think you're looking at between £3-400
You do get what you pay for remember

Mate it to a 4-2-1 manifold and a K&N 57i and it'll run great
