Sorry to post on a dead thread, though if were me if I didn't have a solid mounting service to mount the cabinet too, I would consider using some type of a counterweight system on the back of the unit to balance out the bottom heavy components.. caster are fine on a tile or concrete floor, though carpet is a definite no no!! can be a hazard if you shift the unit full of components..

For venting purposes I would tend to allow for a 2-3 unit per component especially if you plan to house amps in the unit in question, you can never get to much air flow in my opinion, more the merrier is what i say and do..