The -50 is a great large CCD chip, they've used them in the H3DII-50, H4D-50 and now the H5D-50. It's happy place is at ISO50, with 100 as an option, and 200/400/800 as you see fit (which I don't). The -50c is the same Sony chip in the Pentax and PhaseOne gear, with higher ISO being fully functional. If you're outside and need higher ISO, the -50c is better, if inside with lights, the -50 is better.
Folks will push back on the whole MF gear based on their personal belief and value structure. Honestly, if you're already set on the A7r2, you may want to look at the 60/80mp backs, as the difference between 42mp and 50mp is minimal, and you're most likely shooting in studio situations. A perfect pairing would be a H4X/H5X with the P65+ back - there's lots of them out there, it's the 60mp CCD (more of a MF look) and your gear isn't going to be in contention with each other. The big downside is the screen on the back kinda really sucks. But you can get the back and body for under $15k used.
Have you tried out CaptureOne? It's a slick tool, even for Sony/Nikon files, and shooting tethered really works wonders when in studio, so that instant confirmation that your shot is spot on - it works with Phase backs only, so you can use a Hasselblad body and glass, but use a PhaseOne back. The IQ backs add a few features, but if you're inside shooting, or have a workflow that allows it, the P+ stuff is cheaper and just as reliable.
Pentax 645z is amazing, and has a few features that no other 50mp camera has. For starters, it's half the cost (body is ~$8,600) and there are lots of lenses for it. The latest glass is amazing, sharp and weather resistant. The 28-45mm IS lens is stunning, the 55mm is razer sharp and the 90mm macro does all types of magic. There are no leaf lenses native to it, so sync speed is 1/125th. But they've got speedlights that work TTL (even wirelessly). Check out
http://priolite.us/pentax_hss as adding specific lights for situations can be cheaper than a leaf lens system.
What's your timeline to purchase? You may want to try out the gear as part of a workshop with either
http://podas.info/ (tied in with LuLa
or Hasselblad
http://www.hasselblad.com/our-world/workshops . There's even one in my backyard later this year, not sure I'll pony up for it -
https://captureintegration.com/alpa-workshop/It all comes down to what makes you happy - if it's MF gear and the kids aren't starving because of the cost, go for it. Photography is a personal decision, what works for others may not work for you.
-Joe