A basic RZ Pro II kit; body, one back, WLF, and one lens, in excellent condition, should be no more than about $700...tops.
The weight difference between an operating RB and RZ is only about a half pound...6 lbs vs 5.5 lbs. Not that much difference.
The 110mm is a great lens, but I'm not sure the use, or want, of one lens is worth swapping to another camera.
I'm a Pro II RZ'r myself, so I wouldn't fault your decision to swap-up. Lately however, there have been some
great prices on 150mm SF (soft focus) lenses, and also on 127mm f:3.5 lenses for the RB. About $150 ea.
For portraiture with the RZ, I prefer the 127, 140, or 180....but that's me.
If you just want to test the waters with the 110mm or the 50mm ULD, the standard RZ's go for much less than Pro II's.
Remember too, that either of the RZ's have stepless shutter speeds when used with an AE prism in aperture priority mode.
Portraiture with flash or strobes is still done manually, w/WLF, just like with your RB, as the only way I know
to do automatic ITTL exposures with an RZ is with an expensive, and/or hard to find Metz flash system,
dedicated specifically to the RZ, and with an appropriate AE prism attached.
The AE prism's can be used for automatic exposure control in natural light and hot light,
(continuous lighting) portraiture, still life, street, or landscapes.
So, if flash/strobe portraiture is your main game, the half second shutter speed of the Pro II could be of [some] use to you.
Otherwise, try the standard RZ. You should be able to find a nice 110mm RZ Pro kit for around $400-$500.
Some other notes on standard RZ Pro, to RZ Pro II compatibility:
The AE Chimney finder only works on the standard RZ Pro, not on the Pro II.
There are separate AE prism's. One for the RZ Pro, and another for the RZ Pro II.
The Pro II prism will work on either RZ, but the prism for the standard Pro won't work on the Pro II.
(MAC did once perform up-grade modifications on Pro AE prism's for use on Pro II's, but I think that service has ended).
Same story with winders, Pro for Pro, Pro II with either.
Besides the half second shutter speeds, the Pro II also has a fine focusing knob and gear, which is easily [and usually] stripped.
With film backs, I have had better luck with the Pro II backs, however, unlike the differences identified above,
any RZ film back will work on any RZ body, though, the inserts from the backs are not interchangeable with the opposite shells.
Pro inserts only fit in Pro magazine shells, and Pro II inserts in Pro II shells.
Unless I forgot something, basically everything else about the two RZ's is the same.
Tilt shift adapter, focusing screens, WLF's, grips, extension tubes, tele-converter, and the whole set of over 17(?) lenses
works on either Pro, Pro II, and Pro IId.
The Pro IId will accept, IIRC, only two (2) specifically designed digital backs, directly to the camera body.
Any other backs will need an adapter plate just like the other RZ's need when attaching a digital back.
I don't have a digital back, so I will let those that do, [Own-and-Use digital backs on the RZ], elaborate further.