I consider the very early Asahi Pentax and later Pentax 6x7 cameras, from the late 1960s to early 1970s, to be the least best or wisest choice in terms of long term serviceability/reliability. Angling for a 1989-era Pentax 67 is a vast improvement, but a lot will depend on how the camera has been used, either professionally (common to see beat-up versions operable for a period, before one of the common age-related problems spring up) or amateur use (neglect, careless use and long periods of no use at all, which is not particularly good for any of the Pentax MF cameras). Review what you see available out there with care and get a camera in your hands to examine.
The Achilles Heel with the Pentax 6x7 / 67 cameras is of course the meter coupling chain. Clue up on the correct method of removing and replacing the TTL meter prism (and only that prism) to avoid this. Other problems include off-cal TTL meter (under-, mostly, but sometimes over-exposure), derangement of shutter speeds, shutter drag/uneven spacing, wind-on gear pawl failure, sticky/slow mirror counter roller failure, mirror solenoid sticking (requiring either service or replacement with another, likely old or more recent specimen, plus reassembly and calibration of the lens mount for focus accuracy). The list may seem to go on and on, but 20 years back I had much, much more problems with a 500C/M Hassie's body (not precisely square), the flaky shutter and the lens lock. It just did not go well with me. So I made do here and there with a Mamiya C330. I will put it very blungly: I would never recommend one of the old Hasselblad cameras for a first-time user.
Light seals are an absolute pain if you are replacing them on the prism; they will need replacing one day. Body seals (there are not many of them at all due to clever channels that do a great job of sealing out light) hold up very, very well unless the camera has been exposed to very high temperatures that causes the material to literally melt, with one consequence being that molten goo finding its way onto the shutter curtain!
Chimney (with its own adjustable focus!), waist-level, right-angle finder (with a wide range of adjustable dioptric correction on it), central-area eyepiece magnifier and even no prism at all-focusing provide plenty of options!
Link to Sticky: Pentax 6x7 / 67 (all variants) meter coupling chain - PentaxForums.com