Hps are alot cheaper to run that that. a 400w HPS on 18/6 for three months would cost £35-40, would you call that alot, I don't. I have a very similar size grow space to you and the only real heat problem is the heat underneath the light, it literally burns the ground so you have to watch out. If I was you I would make a hole in the top of your closet so you can install a inline extraction fan connected to a carbon scrubber. They are expensive. But you can pick up the fan, the scrubber and ducting for less than £200, and that will sort your heat problems, your ventilation problems, and your odor problems all in one and no maintenance. So personally I would find the money and do that. If if your strapped for cash then there's not much you can do to tackle a 400w HPS except not use it.