Besides sheer image quality, a big gun would help me set myself apart in a market where most photographers use DSLRs
You might want to rethink your premise. Clients, for the most part, don't know or care what you use. You'll stand apart by creating great work that's unique. All the pixels in the world won't help you with that. Besides, it's a 72 dpi world out there on the web so no one can tell what you're shooting when they see your web site.
I'd ditch the RB system. I shot with one for years (film) and was thinking about buying the digital adapter for it. The RZ is a much better system and is really cheap these days. There's all kinds of used stuff out there. A body is a couple of hundred bucks at
www.keh.com. I've got an RZ adapter for my back but I don't use it much. I like the Hassie H set up. It's the best option for my style of shooting. It's not cheap though.
I wouldn't bother with the Mamiya ZD. It's 12 bits, slow and, from the reports here, has issues. I'd say you're better off with getting a real MFD back off of ebay. You'll have to be patient, but there's some good deals out there. The new stuff is getting cheaper by the day so that's going to downward pressure on the used market's prices.
If you're scrimping and saving to get into the business, there's probably lots of other ways to invest your money that will be more productive. Mediocre images produced on high resolution backed by inadequate marketing will get you nowhere except broke. Focus on your craft and your marketing first. The expensive gear can follow when you have the clients ready to pay for it. My obstreperous two cents...
John
