You need to consider a number of things, here are a few:
1) How long it takes to get to speed - BIG motor required. Hypnodisk was modified later with more power for faster spinup.
2) Perfect balance of the rotor - or it will shake the robot to pieces.
3) Strength of the rotor, it musn't lose balance after a strike (seee above).
4) Strength of the entire mechanism - it must be able to withstand the massive shock loadings.
5) Some kind of safety drive mechanism (a clutch or something), so that when it hits an object, and stops dead, it doesn't destroy itself.