There's quite a lot of choice, try and go for a full stainless system, which will be everything except the downpipe from the manifold (which are mega expensive in stainless) and expect to pay around £300 - £350. There's quite a bit of choice, myself I got a Scorpion system with the twin DTM backpipes on, which I like. But different strokes for different folks I guess.
You'll be looking at a few extra bhp, more if you get a downpipe, and a much nicer sound, but be carefull cause some are freaking loud!