Since the activated ability doesn't have a tap symbol in its cost, you can tap creatures that haven't been under your control since your most recent turn began to pay the cost.

The power of the creature you tap is checked as the ability resolves, not as the creature is tapped, and not at any later time should the power of that creature change. If that creature is no longer on the battlefield as the ability resolves, use the power it had when it was last on the battlefield.