I don't know what prices are like for you but I'm surprised that you say a Mamiya 6 is too expensive but then are thinking about a 500CM.
In Tokyo the 6 is no more expensive than the blad. And if you want portability, the Mamiya is the clear winner!
The cheapest camera I have that fits your description is the Bronica C, which I pulled out of a junk bin for next to nothing. It really is great! It is also cheap even in good condition because it was a budget model to begin with. The drawback is the absence of exchangeable backs.