I am happy to report that this project has finally gone forward!
I got bored of looking old server cases from eBay (and the postages would have been astronomical). So I checked out my nearest PC- superstore, and bought the biggest possible ATX- case, the model I chose was a
NZXT Switch 810.
Although being a VERY large case, the motherboard only just fits in width! I had to drill new holes for the motherboard stands and cut some metal out of the rear panel. With a help from a friend from work, the board is now installed in the case, and the system has been quickly tested - and it works!
Pictures available
here.
Some more work is needed around the rear connectors (I have a piece of white, easily cuttable plastic sheet of which I'll do a plate to cover the rear panel connectors) and cables need to be routed but it's already looking like a real computer!
The PSU can be fitted at the bottom (doesn't quite fit the opening in the case but fits inside the case). I didn't even try to make the Zorro slots really usable (or to fit the slots on the rear panel) but at the moment I don't have any Zorro boards for this machine, anyway. If I someday decide to expand the system I have to come up with something.
The case came with four 140mm fans, I'll see how to put those (and I think I'll need only two or three for this system anyway).
One must be real crazy to have a hobby like this...