Knowing the mass, or more accurately the moment of inertia, only relates the torque to the angular acceleration but once the fan is spinning at constant speed, the mass is unimportant.

What matters when running is the difference in air pressure from the inlet to the outlet and the flow rate through the fan. Those determine the power you need from your motor which will also be the torque multiplied by the angular velocity.

You will have to allow for a sensible efficiency too, it's not my area of expertise but roughly you'll get:

torque = (pressure difference * flow rate) / (angular velocity * efficiency)