I have an RB but I can't say whether the 645 prism fits. If I had to guess I'd say no, the 645 should be a smaller system. You can get a prism finder for the RB. The lenses definitely won't fit. I think.
Edit; I'm gonna say no and no. Even some of the later KL lenses won't fit the earlier RB bodies cos they changed the diameter of the mount/rear flange, so if you're thinking of a body, better to get the later Pro-S (or the Pro at least). The earliest lenses are single coated (non C), the next C lenses are multi-coated and better, and the KL lenses are excellent.
It's often a good idea to buy this type of system as a complete kit, as it's getting more and more difficult to come by second hand equipment. Usually the more gear being sold at once, the better the deal.
And I have a question. I have a heap of old slide film that's years(?) past its use by (but probably still OK). Does anyone have any ideas for something interesting to do with it? I was thinking of heating it up a bit to make the colours change. Perhaps just cross process it. I have 9 rolls of Provia 100F and 1 roll of 400F. Anyone know of any cross-processing methods for Provia? Or something else?