The 900 should be fine for that commute. I can't speak to the tail light. Your speed should make the time plenty adequate for the battery life, though I'm not sure how much running the tail light off the same battery will shorten that. I use separate tail lights (2xPBSF's+RadBot).

The 1400 seems silly.

Also, it's not 1400 watts. That would cook your handle bars as well as you.