No wood. I cannot stress enough how bad an idea it is to make a PC case out of wood.
In theory, Designing it will probably take the longest amount of time. You have to take into consideration PSU's for each of the machines, Motherboard trays (would probably best if you could rig these so they could slide), Drive trays, Riser cards for GPU's etc.

Personally, I'd build it as a staged system. PSU's on the bottom, vertically alligned Motherboards on trays, followed by Drives. Personally I'd leave the optical drives IDE & power cables exposed, for easy hot-swapping between machines.

I'd expect this sort of rig (if the framework is well designed) to be about three times as wide as a standard ATX case and about similar height to a Full ATX case.

Good luck and post up the designs if you ever make it :-D