As I see it there are lots of compact options with interchangeable lenses, I don't know of any you could get for $500. Last time I looked at the Bronica 645 rangefinder and Mamiya 7 they were well over this budget (an M7ii with lens was over £1000 last time I looked).
I'm guessing for $500 you could get:
- A Mamiya C330 with 2 or 3 lenses
- A Mamiya 645 Super/Pro with body, lens, prism, winder and back
- A Bronica SQ with body, lens, back & WLF
My favourite would be a Pentax 6x7 but I think you'd struggle to find one for less than $500. I'd prefer a Bronica SQ to an RB/RZ67 unless I wanted it for studio work (the RB/RZ67 are really 7x7 cameras/lenses and much bulkier than a 6x6).
John.