Mamiya RZ67 Pro IID or Hasselblad 503CW?
Personally I'm a fan of the Mamiya but your question is way too subjective for an answer. See if your camera store will let you run a roll of film through each one. Or see if there is a camera rental place near you where you can use each camera for a...
Where can I get an adapter to use Mamiya medium-format 645 lenses on vintage Mamiya 35mm cameras?
I'd try www.bhphotovideo.com they have just about anything you may need.
Can Mamiya RZ67 (original model) Medium Fromat Camera use the RZ67 PRO IID Lenses?
That would stand to reason. If there is some quirky thing that would not allow that, I would think you would need to call Mamiya or just go into your local camera store and ask.