You're probably in cat-back price range there rather than turbo-back, so don't expect any big power increase. Replacing the downpipe, and cat (with de-cat or sports cat) is where you'll release some power.
BCS are very popular on here, they do a Powervalve system that is quiet and civilised at part throttle, then nice and meaty on boost as a valve opens that bypasses a silencer.
Milltek are also a tried and tested brand. A few used systems come up for sale on here, and a full turbo-back exhaust with bigger downpipe may well come up within your budget.
Between the two, the consensus is that BCS are possibly better quality, but Milltek are still very good. If you go for Milltek, their 'resonated' systems are a bit quieter than their 'non-resonated'.
Hope this helps!