Bicycle Mechanics - Converting from 650c to 26" questions

Question one: OK, so I got stupid and bought wheels before I examined my frame closely. The wheels I got are Grimeca-Razza cast MTB wheels for disc brakes. I can get a front fork with a disc tab, but my seat stays have no disc tab at all. Can I run rim brakes using a cast wheel made for discs? Obviously, it can be done, but is it wise? It would obviously ruin the paint job on the cast wheel at the very least. Would it be better to just resell the cast wheels and buy conventional wheels for rim brakes?

Question two: I want to convert a bike that originally used 650c wheels to 26" MTB wheels. To do so, I'll need a 1" threaded fork with V-brake bosses and a pair of MTB wheels (ULTRA heavy-duty ones) for 9-speed Shimano cassette. Looking on e-Bay, I find almost nothing but suspension forks (I want a rigid) with nothing but threadless fork tubes. Where do I go to find the parts I want? I looked at Nashbar, but all their forks are threadless or suspension too. A used fork is fine by me, and I'm not eager to part with a whole lot of cash. I thought I might be better off buying a complete older-model mountain bike at a garage sale & stripping just the stuff I need? Have you any other suggestions on where to find such swag?

I would reconsider the whole project and probably sell the 650 to fund purchase of a lightweight MTB frame.
Bikes are only as strong as the weakest part. Beefing up the wheels and fork will leave the frame as the weakest component. If you fit havyweight MTB fork and wheels to a lightweight 650c race frame, if you take a hit at the front the part that bends is the downtube of the frame. If it has the correct fork and wheels, the fork would bend and save the frame from damage.

...If you fit havyweight MTB fork and wheels to a lightweight 650c race frame, if you take a hit at the front the part that bends is the downtube of the frame...

Not an issue - this is a recumbent that will be used only on road.