Example comparison here.
Swapping backs, that rules out Pentax 645 and 67, you've said Contax is too expensive, so all that's left is M645 and RB/RZ.
AF Confirm, that's pretty much M645AF (or the already ruled-out Contax, not sure about Pentax).
Lenses, M645 AF lenses are cheap, MF lenses are cheaper (except the 80/1.9 never got made into an AF version). About as cheap as you'll get in MF lenses except Soviet K60/K88 lenses which are a crapshoot (trust me, been there). Also as I've said, adapters let you keep the current ones you've got, even if only to use before you get native-mounts, and then IQ is exactly the same as before.
Fisheye, look into the Zodiak / Arsat / MC Arsat 30mm f/3.5. Blatant ripoff off the $2500 hassy, maybe not as good, but definitely only 1/10th - 1/20th of the price. Pentacon 6 and Kiev 88 mounts natively, maybe they made them in Hassy, definitely adaptable to any 645.