I'd probably pick up a Pentax 645 (the Manual focus model), they're much cheaper these days. A camera with 120 holder and 75mm lens probably goes for $250-300 these days. Maybe less.
The autofocus models fetch a bit more; probably $400-500.
For roll-in-the-dirt, take a beating cameras, a Holga or maybe an older Kodak Brownie should do it. Limited exposure options, and you'll need gaffer tape to tape the backs (and red windows) closed, but there's really nothing that can go wrong in them, unless a spring breaks or falls out.