There are three main parts to a wheel to consider: spokes, rims, and hubs.
The biggest consideration when using disc brakes, is the hub. the hubs need to have a bolt pattern that matches the brakes you want to use. (More on brakes in another forum)
Note the hub below does NOT have a bolt pattern to attach a disk, but threads for a drum or sprocket.
The next consideration is how many bearings. The more the better. Ceramic are better than ball bearings. Consider that cost increases as # of bearings, but so does efficiency. There are very good hubs out there with only two bearings at a very reasonable cost. Anybody else have any specific hubs they would recommend for cost efficiency?
Here is a cut away of a 3 bearing hub:
Also don't forget to consider the size of the axle. If your axles are not metric (most hubs are) you may have to go to Grainger and swap out the bearings to make it work with your axle. One of our cars we actually had a sponsor make aluminum sleeves (bushings that fit between the axle and the bearing surface.
All this said, we use 2 bearing Hope hubs with MTB brand brakes with 180mm disks. These are pretty economical, and readily available at most bike shops.
Rims & Spokes in another forum!
More great info in the Electrathon America Forum including types of rims, spokes, and hubs used by others!