Attaching a digital back to a 6x7 body is pretty much pointless, since even a 50mm lens won't be really wide angle with even the biggest digital sensors you can buy at the moment. The biggest advantage of the Pro IID is the fact, that it's most likely newer and as a result in better shape than a Pro or Pro II. One shouldn't forget that bot RB and RZ were serious professional cameras which saw heavy use. If you go to a used camera store, you'll be amazed how scratched and dinged some of this equipment is. My used Pro IID was marginally more expensive than comparable Pro II offerings and it looks and works like a brand new camera.
I haven't yet come across any used IID versions yet, http://www.ffordes.com have used ProII's which look in good condition however these come to about £800ish and im sure that I could find them cheaper than that!
The IID is much, much more expensive than either the II or the original rz67 solely because of the digital back compatibility. If you don't need digital compatibility, get an earlier model. The original is about $200-$250 used and has 95% of the IID's functionality though it might be harder to find one in very good condition.
I have an original rz67 and it's fine, but it's not light. I don't think it's much lighter than a 4X5, which, to be honest, I might rather have... The lenses are very heavy, too, and heavier than large format lenses of equivalent focal lengths.
I use mine mostly for landscapes so far, but the Pentax 67 and Mamiya 7 both have wider lenses and might be better landscape cameras. I chose the rz67 over the rb67 because of the 50mm uld (which is reasonably sharp and generally just nice) and the 110mm f2.8, which is remarkably sharp, even wide open.
That is my mistake. After a little bit of research, I realize that I should have said that you don't compose the entire shot if you use the standard 6x7 focusing screens. You'd need to get the 6x8 focusing screen to show the entire view. I believe you also need a 6x8 film back adapter (which also works for 6x7). I do not believe that Pro SDs (with their 8x8 cm rear aperture) have quite as many issues to consider when using the 6x8 back. I will defer to the experts on 6x8 backs to explain the details of their use in detail.
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