Yep a 400w will do fine, how many plants are you growing? A 400w HPS can do around 5-6 plants if youve got mylar, not sure on how much better/worse a MH is though.

If you use the HPS for lighting at the side during flowering, the buds will be a little denser and theyll grow a bit bigger aswell. You shouldnt have any heat probs with a 150 either, my 600w HPS doesnt actually produce that much heat.
Unless you touch it.