My experience may be helpful, I have a P6x7 outfit with a few lenses and the 645 adapter (I had already envisioned renting a 645Z at some point - the practicalities of renting the gear with a 100% credit card deposit as an amateur ended that idea!). I've shot loads of roll film in the past in 645, 6x6 & 6x7 formats, not necessarily tripod mounted but I'm comfortable with slow film and slow working. Six months ago I bought a mid-milage 645D for similar money to a K-1ii from a very good Pentax specialist dealer with a year warranty. I've never shot video, used LiveView or high ISO on my K-5, the 'limitations' of the 645D are not significant to me (I did buy a right-angled viewfinder attachment for 'waist level' shooting due to not having a flippy screen - it works well).
I shoot it like an MF film camera - slowly concentrating on the images, rarely shooting more than a dozen frames in a session. The form-factor of the camera is so close to a typical MF camera, it encourages me to shoot it the same way. All of the P6x7 lenses I've tried have produced good or better results (I'm not worried about scientific levels of sharpness, the larger format is far more important). The limitations being wide-angle focal lengths - the P6x7 45mm is good but only equivalent to around 35mm (FF). Since buying the 645D I've also bought 33-55 & 45-85 645 AF zooms and prefer these to P6x7 lenses for the convenience of them having a native mount. The 33-55 has some negatives according to the reviews, but it's the only wide-angle I can justify and I needed the extra width - in practise, it works well for my needs.
Although I still shoot MF film, getting high quality scans is expensive and considerable hassle (I need a better scanner and computer), the output from the 645D obviously eliminates this. Print wise, I've been experimenting with lab produced A2 prints (the largest the 645D will go at 300dpi) and there's easily enough detail in them for me.
Since buying it I've also bought an original K-1, but my preference will always be the 645D when I want to get serious. Medium format digital always seemed way out of my reach and totally un-justiable - a used 645D (and some P6x7 lenses + adapter) suddenly made it do-able. Having accessories common to my other Pentax cameras (batteries, chargers, flashguns, IR remote controls) really help reduce the overall cost when considering it as a 'system'. It's perfectly feasible to carry only P6x7 lenses, the adapter and both 645D & P6x7 bodies to give the best of both worlds, a 645/N(ii) would work similarly with native 645 glass.
I'm very happy with my choice and the results it gives.
John.
Last edited by johnha; 11-29-2018 at 04:58 PM.
Reason: Updated comments about the 33-55