That's a real small space to put a hid in, I take it the height is 3'? Allow 8" for the pot/s, 6" for the space the lamp and reflector take up and at least 8" distance from the plant to the light, unless you shell out for an air cooled reflector. That leaves just 14 inches for your plant/s to grow in. You'll have to go 12/12 from seed/clone and train extensively to keep them from getting burned, even with kick-ass ventilation. I'd build a bigger box personally.
If for whatever reason you stick with this, try to get a cold air feed directly into the box from your ac, and make sure the exhaust goes out of the room, outside preferably. You'll want a good 100-150cfm fan, and get a centrifugal, or vortex type fan as the inline axials will be defeated by more than a couple of feet of straight ducting.
Whatever you decide to do, good luck and happy growing.