Hi everybody,
Thank you all for your kind advice and apologies for my delay in responding.
Lots of information to ingest here, and a special thanks to Synn for the wonderfully put together details on compatibility.
I bought my Mamiya 645 AFD III based on the belief that it provided the best balance of lens/back compatibility for the price (I got the body with an 80mm lens for £500), but then it seems I would need to buy a back that really lacked any future proof should I upgrade to a Phase One XF system in a few years, but as Synn pointed out, if I do this, and can afford it, I would likely care little about losing some money on the resale value of my old 645 AFD compatible digital back.
I also have an RZ67 Pro II to which I am very new but have done some shooting with, with some very pleasing results so far (for me at least; a baby amateur by a lot of your standards!). I was hoping some level of compatibility between what I buy for the RZ67 Pro II and what I buy for the 645 AFD III, but it looks like that may have been a mistake; if anything I can get an H adapter for my RZ67 Pro II, potentially buy a Hasselblad H2 and be able to share my back between the two bodies, and if I upgrade later to an H4X I think would still work on that, if I would be unable to afford to upgrade back and body at the same time.
I haven't really looked in any depth into the lenses for Hasselblad yet to see the price difference people talk of, but I have read a number of reviews of various newer(ish) Phase One and Hasselblad systems and it does look like both are extremely capable systems that would be limited for some good years by my own ability as a photographer more than their own quality as systems.
@leeonmaui, that is a great price on the Pentax 645D, thank you for the link! But it is body mated and I definitely want to maintain a modular system for now.
One thing seems clear: I have probably bought the wrong body. Then again, if I can find a cheap enough back to go with it, I can begin collecting some Phase One lenses that will work down the line on future bodies and don't need to worry too much about the resale value of the (relatively) cheap starter equipment.
I've written all this and still feel confused