Manual trans swaps seem to fetch $5k for trans, clutch, pedals etc. I imagine you could negotiate a 1:1 swap with someone, but wading through the dreamers will be tough.
Lots of people seem to be opting for the ZF 8 speed swap these days. That's an expensive swap...
The only other thing to factor is that it's a fair risk buying a used ZF. In principle, if you could find one from a wreck with low km maybe it'd be ok, but lots of worn out ones out there. And most people that get them reconditioned seem to have issues.
I think your idea is sound. In Melb you could have a chat with Max Performance to give you a ballpark. They might even recommend someone else.