Front-surface partial mirrors get cheaper as you go smaller; I'd suggest that medium-format is a good place to start. In particular, using RB backs gets you a simple (graflok) mounting system for your films.

The real problem (unless I'm missing something obvious here) is that the mirrors will take up a lot of room in the optical path (about twice as much as in a normal SLR, and that's without any SLR viewfinder!), which means you're limited to quite-long lenses. It will be difficult-to-impossible to build a wideangle camera, and you'll want to have lenses with a registration distance as long as possible. You may only be able to support quite narrow fields of view.

Quit worrying about thermal expansion; such details are an irrelevancy.

You can get free CAD programs. Hell, you can use google sketchup or Inkscape with zero training and a little practise. Design the camera to be built entirely from shapes laser-cut from flat material, make yourself some drawings, send them off to ponoko and they'll send you all the parts in a couple of weeks. If complex 3D stuff (cams, etc) are required, they can be 3D-printed.