I searched for 'kitcar exhaust' and got a Westfield Megablade one. It's very small, the size of the bike's original can, but it's reasonably silent. The bike had ~160 bhp, so should be fine for my Zetec, too.

Not something I have thought about yet but I'm guessing as the exhaust will be a lot shorter and only have one silencer they will all in theory be less restrictive than any road car exhaust, just need to make sure the noise level is acceptable.
I don't know how much choice there is on pipe diameter or what affect it would have.

The basic Robin Hood can wont pass IVA without a Db killer

That's how I got mine I serched kit car exhaust you could also try zetec exhaust or something like that. I got the full system from an mk Indy it only needed slight modification to fit