Well if you don't mind a bit of weight a Mamiya RB is a great choice and purely mechanical. The camera uses interchangeable lenses, interchangeable finders and interchangeable backs. Just treat the 6x7 frame as 6x6 (even draw lines on the ground glass) and if you want 645 get a 645 back.

Here's a current example of one for sale right here on APUG.


If you want a metered prism they are available but I'd suggest getting a handheld meter.