Sadly mate, you say you don't want a cheap one, but £75 is stupidly cheap in the world of modifying Audi's!
On your 1.6, no backbox is going to add any power at all. It'll just make a nasty noise. I had a friend with a backbox on his 1.6, and it made the car worse to drive, it felt gutless, and made a horrible drone.
If you want it for the looks, then that's cool.
Search on the golf forums for a milltek catback, they tend to go for about £150 for a used one. the mk4 golf exhausts sit much better on the A3 than the A3 designed ones too!