Answer 1: There is also gyroscopic force which is a function of inertia and speed. This is the force that keeps bicycles, motorcycles, etc. from simply falling over while they are moving. Even a small wheel rotating at high speed will generate forces that you can feel. The classic experiment is spinning a bicycle wheel while hanging onto the axle. As you try to steer the wheel you can feel a resisting force trying to keep the wheel straight. The faster the wheel spins, the more force is required.
Answer 2: In general, for a wheel to rotate a such a high speed, you will want to concentrate mass (presumably the steel, so
small OD
for the steel parts) as closely as possible to the motor shaft and minimize the overall mass of the wheel to only what is needed to do the work you need to do. Also, the strength of the materials comes into play. The farther away from the motor shaft the higher the force will be to pull it apart (a=v^2/r). Note that the force increases as the square of the velocity.