As has been previously mentioned, if you are looking at a CCD used, then consider the 160 (260 is a waste of time as it never really gave a good worth while 1 hour exposure as the P45+ did ) I owned one, I made the mistake of upgrading from the 160. You can tether the 160 to a surface pro 2,3 or current with C1 and get excellent review on the screen of the surface. The LiveView, on any CCD back, is pretty much worthless outdoors even with a huge ND filter installed, but can be used to some degree indoors. Wifi via an adhoc connection for Capture Pilot on the 260 was not the best, never worked that well had a lot of disconnects, slow to transfer.
The 50MP CMOS back, in a 150, 250 (original P1 CMOS back) or 350 would be worth consideration. The 350 might have the same wifi capabilities as the IQ3100 which were excellent for Capture Pilot connection via adhoc. But I don't know if P1 ever upgraded the firmware to allow this. Liveview on any of the 50MP backs is top end, excellent, and review on the same screen is again excellent. 50MP back uses a similar sensor to the sensor on the Fuji, (Fuji used a different microlens design) but it's the same basic chip. Excellent high ISO, and DR.
IQ3100 still IMO was the best back P1 made. Everything just worked and worked very well. LiveView, and preview on the LCD easy to use. Wifi worked excellent, fast solid connection via adhoc and Capture Pilot could be easily used for Viewing LiveView, shutter release, and slow but OK review of the images. The Capture Pilot app, never truly respected the iPhone resolution (respected is a Apple term) so images aren't as sharp as when viewed on a tethered solution. But setting up the shot via Capture Pilot was invaluable. As close to a tilting screen option as you could get.
IQ4, is still lacking any type of Capture Pilot ability, any type of Adhoc connection (odds are never will), less then stellar Live View (doesn't have the same level of contrast as previous CMOS backs at least for me) Exposure Simulation Liveview can't be used in low light due to frame rate and Auto Live View can't handle normal daytime light as it constantly pulses. It's now June and you still can't review a portrait oriented image on the IQ4. But it still has an amazing ability to work with Tech camera lenses, (especially wides) unlike any previous P1 back I know of or have used.
Hope you find a back that works for your needs.
Paul C