I can guarantee most teams will not have calculated the MOI or inertia of their weapons. Most teams just ball-park their system against previous known setups, both ones that worked and didn't.
I'd take the mass of your weapon and diameter and find some robots with similar weapons and see what they use as motors. If you can't afford or have the weight for what they use then increase the ratio so it will spin slower and use a smaller motor.

As for your motors and batteries, they need to be worked out together. You need to guess how much capacity you need and what voltage all your motors run at, be it 24V, 36V or 48V. Again, look at other robot setups, take something that worked and then adjust it to suit your needs.