I am trying to spend under $40 on ebay, which is very doable these days.
I'm sorry to be nasty, but you're absolutely delusional. No one is going to sell you any medium format camera that cheap. You can get a Fuji 6x9 rangefinder for $500 or so, and it does all you want except the ability to change formats. If you're willing to settle on a 6x7, you can get a Mamiya RB67 for a few hundred, and it offers 6x7, 6x6, and 645 backs.
Fine Art Photography of Indiana and other places no one else photographs.