What are your planned main uses?
I'd say the main differentiating factor is CCD vs. CMOS sensor. The most recent generation or two of MF have CMOS sensors; older generations are CCD.
CCD is relatively "light-insensitive" with a base ISO typically 25, 50 or maybe 100 for sensors like the H3Dii with microlenses (and even then I rate it at ISO 80 at best, really). And you can't really push a CCD sensor much past base ISO, in my experience.
CMOS on the other hand usually comes at base ISO 100 and can be successfully pushed, depending on your use, certainly to ISO 800 and in many cases beyond.
As you're looking second-hand you'll probably find many more CCD-based systems than CMOS, and now is a great time to pick up a bargain as early adopters shed their CCD stuff for CMOS.
If you are looking to hand-hold at 1/50th of a second at ISO 800, you're going to want CMOS and the hell with the expense.
If you mostly shoot locked-off landscapes on a tripod with long exposures, you might even find CCD's low base ISO a point in its favour- less need for ND filters.
The Hasselblad system feels like a big dSLR in use and is a nicely rounded system, with good dealer and rental support world-wide. The lenses range from pretty good to very good, and all have leaf shutters (which was a big plus for me as I like to use flash and daylight together on location when shooting models). I've been very happy with my H3Dii although it does have a more-than-usually cropped sensor which is less than ideal if you're after wide landscapes. The big change going to the H4D was a system for correcting focussing errors when you focus and recompose, which inevitably introduces some tilt- the camera corrects for this automatically. I gather it works very well, although at my typical working apertures I can't really say I've noticed the lack. The H5D is a bit of an all-around polish up, with both CCD and CMOS options. The new H6D is a more radical redesign, I gather, and CMOS only.
If my Hasselblad were to suffer an unfortunate accident, I'd probably buy a second-hand H4D-40.
I'd DEFINITELY replace it with a Hassy of some form. (I'd have to weigh the second hand savings against the appeal of an H6D and a second mortgage....)
http://www.procentre.co.uk usually have a bunch of second hand ones in stock.
The Pentax options are attractive. Although I've not used them myself yet, I gather the 645D was quirky but promising. The 645Z seems to be a very accomplished performer indeed, with a CMOS sensor and imaging chain which people feel really gets the best from it. The limiting factor here may be lenses- the quality seems to be much more variable over the range, so hopefully someone else can help here.
Phase backs are extremely well thought of but the bodies they go on rather less so (with the probable exception of the new one). They don't have the "works like a point and shoot when you need to" quality of the Hasselblads and Pentax's, from what I remember of the one time I tried one.
Overall I've been delighted with my Hasselblad, it has been my business workhorse for about six years now. I've supplemented it with a Sony A7Rii for low light.
Hope that helps.
Cheers, Hywel
P.S. in the interests of balance, I would say that the quality of the rear LCD screen on previous gen Hasselblads is truly
awful. Fine for menus and histograms, not really great for checking focus, and totally utterly misleadingly awful when it comes to colour. I hope the later models are better!
