All EV conversions are engineering exercises, and they're almost all done by unqualified hobbyists. The size of the vehicle doesn't change any of the technical issues, but it does escalate the cost (which will be huge).This needs to be engineered and is not a home project.
So forget that it's a bus, except for using the specifications of the bus (weight, tire size, axle gearing, etc) and go through the same options that everyone else does. All three options listed are potentially viable, each with a suitable motor. I think when a motor, controller, transmission, and battery are all chosen, the costs of those components will add up to an unworkable project... unless you're very wealthy and this is where you choose to throw away your money.