Lots of variables there. Generally folks will adapt their grinder(s) to meet their specific needs. If you purchase one of the "top end" machines such as KMG, Bader, etc. they will have a "standard" configuration, but you can add contact wheels as your requirements dictate (at the extra cost of whatever contact wheels you desire). Personally, I don't recommend using wheels other than those made specifically for belt grinder applications. To do otherwise invites safety issues, as well as difficulties operating the machine(s).
Most of the bearings are pressed in on wheels 1" or larger, and sometime on the smaller wheels. (many of the smaller, 1" or less wheels now have external bearings, and require a special attachment to use....these are call "small wheel attachments") There are aluminum and steel hubs available in contact wheels. Aluminum is best on larger wheels to reduce weight. When it comes to smaller wheels, I prefer steel hubs because with aluminum, after a couple times changing out bearings, the hubs become "stretched", making them useless. (the bearing will no longer stay in the hub). Shaft diameter depends on wheel diameter...for example my flat platens use a 3" and a 1 1/2" contact wheel. The 3" wheel takes a 1/2 shaft, and the 1 1/2" wheel takes a 3/8" shaft.
Unless your planning a variable speed machine, most folks go with a 1725rpm motor. If a VFD (Variable Frequency Drive) is used, either a 1725 or a 3450rpm motor can be used.
