Based on 250.104(C),it seems like a good idea to bond the steel work, even though it isn't considered the "building frame".
Is it possible that it could become energized? Bonding would add an additional measure of safety.
Table 430.52 permits the motor branch circuit short circuit & ground fault protection to be set at 250% for inverse time CBs. As long as the equipment doesn't specify fuses, the non-fused disco should be OK.
Are you installing the starter on the roof, or in the building. 430.102 requires the starter have a disconnecting means within sight(visible and less than 50' away). The motor needs one as well, unless the starter disconnect is within sight from the motor.
I would run a separate circuit for the second motor. Too many questions arise, in my view.