The prices posted here for Bronica ETR kits are really attractive, but they are "bargain" condition prices when I look at KEH. Thus, I would expect them to have done lots of hard commercial service. If you plan to add your own hard duty cycle to it, you may find there's not much duty left.
Nice kits in better condition cost more. The ETR with the medium zoom, a back, a body, and a prism are still under a grand, and that's cheap by any measure.
I didn't see much selection of separate backs and lenses, however. Remember that Bronica has been out of production for a while now.
Mamiyas are still in production, and that makes them expensive.
The best balance between those extremes might be the Pentax 645. It is not a system camera like the Bronica. It doesn't have interchangeable finders and backs, but they are still outstanding performers with great optics. And they are still in production. A used Pentax kit that has autofocus and a motor drive is still well under a grand in EX condition, and it uses conventional AA batteries.
I also like the square format. There are lots of options that provide it, from old twin-lens reflex cameras to Hasselblads. One advantage is that you can use a 45-degree prism, which is a really comfortable option, since you never have to turn the camera on its side (or, conversely, worry about turning the camera on its side when needed).
The decision to buy a camera with interchangeable backs should not be taken lightly. Even owners of Hasselblads find that they have to maintain the light seals on the backs to prevent light leaks. And cameras that use lens shutters will limit your options for fast lenses.
Another option altogether is a MF rangefinder. The Fuji G-series fixed-lens cameras are just outstanding, and some have zoom lenses. They may look like big point-n-shoots, but they were used by working professionals to get high-end results, and loved for their ease of handling.
Today's prices are amazing. I think of the thousands I have spend on medium-format stuff and how all that is worth only pennies on the dollar now, and I wonder why anyone would buy a new Seagull (or a new Hasselblad). I also wonder why camera manufacturers are still making them. I don't like what those wonderings may portend.
Rick "who thinks 6x7 and bigger makes a real difference compared to 645 even at print sizes smaller than 20x24" Denney