1) I'd go with a 3-4KW spindle, as long as you get the Nema 34 motors.
2) You'd need to ask Avid about that. I would think it would be a problem, but you may find the Z axis will drop with the power off. SO you would probable need to add springs or something to counterbalance it,
3) 18,000 rpm is fine. The 15HP spindle on our Morbidelli is only 18,000, and we cut 5x faster than you will on the Avid.