600's tend to be the most energy efficient of the hps models and the heat is way less than the 1000's. Could probably grow 12 plants efficiently with that size area depending on how big your going to let them grow. If you've got indica's about 3 feet tall works out good.
The best way for venting would be to vent in the attic and either hook into a carbon scrubber up there or put a UV ozone generator up there.