One of the things I've been considering doing is getting a magnaflow cat but I don't have any idea how it would sound and also if it affects emissions as if it does then I won't be passing the vehicle inspection lol. If you look up magnaflow 323f on ebay uk alot of results will appear, all with different numbers which I have no idea what they mean apart from maybe diameter? If so that also leads me to the question of what diameter would I need?

You're best option would be to head down to your local exhaust specialists and get a sports cat welded in. I wouldn't trust any of the sellers on eBay for a 20+ year old car parts, can guarantee that 99% of them either won't fit properly or be for a completely different car