I'm not clear on this statement.

"they are rim brake rims, and there is very limited demand for them now, as everything is disc."

My bike is disc brake and I think I can use any type of 26" wheels as long as the hub is disc brake compatible.

I want to use my mountain bike to be able ride in the track and I do not want to buy another road bike for it. So I'm thinking about get a road tire for it. 700c rims are for road bikes if I am not mistaken.