Thanks again for the replies everyone. I'm not too bothered by the electronic shutter on the RZ. All of my film cameras are electronic and I just carry a spare battery in case one dies.
Polyglot, the 110 lens is exactly why I'm looking at the RZ.
So my one remaining question is about the Polariod backs. There seems to be two of them? The one I can get in the kit I built is the Quadra72 which is a 4x5 back. Is, say, Fuji FP100c considered a 4x5 film? Just wondering if the Fuji FP films will work with the Quadra back. I know nothing about instant photography but it looks like it would be fun to try.