Ewbank,
I had the Pro I, and now I have the Pro II. I checked the owners manual and it says you can use RB finders with the RZ. If I'm not mistaken, you can use almost all of the RZ/RB lenses, finders, film backs with adaptors. However, when it comes to the Pro II you can't use some of the Pro I items like film backs. If you find that you like the RZ, I would purchase Pro II film backs (no foam to replace) If you want to add the AE Prism, get what's needed for the Pro II. Then, if you decide to upgrade the body everything else will work. The RZ is an awesome camera. I sold my complete Pro I system after I purchase the Nikon D750. Three months later I purchase another RZ. This time the Pro II. I have the AE Prism and as mshchem said, it is heavy but, if you shoot color film it's worth it. I shoot only B&W but when using a Cir-Pol filter I'll use it as the camera sits on a tripod. I don't hike with it though!