Welcome to the wonderful world of RZ.

Lenses...a three lens kit is always a nice goal for a start.
Maybe, keep a space of one or two focal lengths between each lens at first, ie; 50mm or 65mm, 110mm, 180mm, etc...
and any of these could be your [must have's], including the 140mm and 210mm. (Opinions & bellybuttons...everybody has one).

Film backs...having two or three 120 backs is more useful, (my opinion), before buying even one 220 back.
Not that much 220 film left.

The 6 X 6 back is pricey, and a bit rare. It was only made during the early years of the original RZ Pro,
mostly for slide presentations. Slide projectors for square, 6 X 6 MF slide mounts, were available from multiple manufactures,
but projectors for rectangular, 6 X 7 mounts were rare, and very, very expensive. Still are.

The 6 X 4.5 (645) backs are useful, especially when trying a new lens or technique, bracketing, etc...with 15 shots a roll instead of ten.
Some criticize this theory, saying, "If you're going to pack around the heft of a RZ, shoot the full 6 X 7 frame."
I don't shoot my 645 back very often, but far more often then my 220 back. May be that's just me.

Prisms & Winders...the RZ Pro II, requires it's own, dedicated, (and usually more expensive) version of these two accessories.
Unless modified, the AE prism from an original RZ Pro, won't work on a Pro II.

Fine-focus knob...the fine focus gearing on the RZ Pro II, can be easily stripped. As the name implies, only use the fine focusing knob
just for that final, tiny little bit of bellows movement. Turn the fine focus knob...slowly, and...sparingly.

Enjoy,
Marc