The GS645S is the follow-up to the earlier folder, and has a 60mm f/4 with the protective bumper. I've had one for several year, actually my first medium-format RF, and it is a decent economical choice. There are upsides and downsides.... First, the lens is just wonderful. The camera is extremely light weight and compact, easy to carry. ergonomic to hold. This is because it's mostly plastic, and the internal lens mount pieces are a bit delicate (see above about the bumper!) The viewfinder is great, with framelines that not only move for parallax compensation but shrink too as you focus closer, for more accurate framing. No interchangeable-lens RF does this field-size change, to my knowledge. Coming from focal-plane shutter cameras mostly, I find it awkward to deal with concentric shutter speed, aperture, and focusing crowded all on the lens barrel. If you expect the leaf shutter would be whisper quiet, you're in for a surprise... There's a noise-maker built into the shutter release and makes a loud "CLACK" as you trip the shutter. Using the self-timer, the muted leaf-shutter "click" is delayed after the CLACK. All in all a nice travel camera, easy to have with you all the time, and you'll love the pictures... but be very careful not to knock the camera against hard objects.
There's also a GS645W with scale focus and a 45mm f/5.6 lens. I also have a Fuji GA645Wi, its replacement, much larger and with pop-up flash, motor wind and lens extension, AF, AE, etc etc like a giant Point'n'shoot. Mine has a 45mm f/4 but there's also a version with 60mm f/4 and a later model with 55-90 zoom.
I have a pair of Bronica RF645 cameras too... Compared with the Fuji GS645S, this is a larger heavier more solidly built camera. It also has interchangeable lenses, with the standard 65mm f/4, optional 45mm f/4 and 100mm f/4.5 (which is rare and now insanely expensive). The rangefinder is great, big and bright, with parallax-compensating automatic framelines for the 65 and 100mm lenses. But no field-size adjustment as you focus. The 45 comes with an accessory viewfinder, but I just look at the entire viewfinder area as a good approximation (not parallax compensated of course). Due to this issue, I tend to prefer my Fuji GA645Wi for wide-angle use, besides it focuses closer too. But then I've been doing environmental portraits close-in and indoors, exacerbating the parallax error.
Ok, within the 3-lens limit, the Bronica RF is sort of a medium-format Leica. With a built-in anatomical handgrip. And matrix meter but not TTL. Very quiet and unobtrusive. Quality construction. Company no longer makes cameras, maybe Tamron regrets acquiring them. Maybe the last and best medium format RF made... Offeres Program AE, and aperture-preferred AE, and metered manual. Fuji GS is just metered manual.
Hope this will help you weigh the priorities... 🙂