I don't know if the Hasselblad really belongs in here. It is a different frame spacing than the other two, and is much smaller.
So, I think the first thing you need to do is pick a frame spacing.
IMO, use a Hassy if you like square format. Use a 645 or a Hassy with a 645 back if you want a camera with the size, convenience, and ergonomics of a Hassy, but don't want to print square format. Use the RB or Pentax if you want to make prints at off-the-shelf paper sizes, and do not need a small camera like a Hassy. Hassy's only big disadvantages to RB/RZs and Pentaxes are:
1. Frame size. This is a very big deal IMO, unless you want square prints.
2. Slower to use for close up work
3. ...and of course, price. Price of EVERYTHING. Parts, accessories, lenses, backs, repairs, etc.
Other than those, I would use a Hassy instead of an RZ and M45.
For the 6x7 choices, the things you need to look at involve the main "operating differences" between the systems, as you will obtain quality results and have quality lenses with either.
The things that are BIG differences:
1. The Mamiya is a full-on component system camera, with exchangeable magazines. The Pentax does not use magazines. There are pluses and minuses to each of these. If you don't need magazines for what you do, the Pentax will require less service and the film plane will always be in exactly the same place. However, the advantages of magazines are so numerous for so many situations, that if you can make use of them, they outweigh this, IMO.
2. The Mamiya uses a leaf shutter. The Pentax uses a focal plane shutter. The Mamiya synchs at all shutter speeds, while the Pentax does it at '30 maximum. This is important especially when mixing ambient light and flash. Otherwise, I see little difference, as long as you can shoot in a dark studio.
3. The Mamiya has close focusing ability without using extension tubes (though extension tubes are also an option). The Pentax requires extension tubes to focus close. If you shoot close (but not necessarily "macro") a lot, big deal. If not, then no big deal.
4. The Pentax is nice and simple ("brainless") to use. The Mamiya is more versatile, but requires more attention to use.
The final opinion: If you don't mix flash and ambient light, would not make good use of magazines, don't focus close often, and have trouble with technical matters (and there is nothing wrong with that - technical matters are just something one must deal with to get what one wants artistically)...in other words, if you just want a nice and simple "big 35", I would say that the Pentax is the better choice. If you want an almost infinitely versatile tool to cover a huge array of situations, specifically mixing flash and ambient light and using magazines, the Mamiya is the better choice. Even if you are technically inept (as are many of my friends with both RB/RZs and Pentaxes), practice does make perfect, and repeated use of the camera will get you up to speed.
As for lenses, forget about it. They are all good.
As for hand holdability for "technically sound" shots, don't expect it with either of these if you are using long lenses and/or shooting in low light. The WLF will help a bit. There is a WLF for both of these cameras. However, I would say that if hand holding will be your primary use, there are better options than either of these (such as Hassy if you want an SLR).