You could get away with a 600w bulb if you installed rails so you can move your light.. Or you could use 2 400W bulbs..

It looks like all 12 buckets would take up that entire grow box, so you might want to consider something with a switchable ballast.. So you can have your HPS and MH in the same socket.

Personally, i would split the room in two, so you can have a veg side and flower side. that way you dont have to wait 4-6 months between harvests.. smaller harvests, but more of them.. the choice is yours...