A halide? I am not sure that will do what you want unless you are talking about MH. As far as fresh air intake, when the temps are low you should just be able to recerculate the air around in the room and it will stay pretty fresh. The hot air holds more properties that are dangerous to the plant, but once the temps drop allot of those properties become dormant or just die off. I would just use a oscillating fan if the temps are to low outside you don't want to bring them in and risk freezing your plants unless you have a active intake fan bringing in the air with a real high out put HID running at the same time and that will off set the cold, and then it could shut off when the light go's off. You should get a box heater, maybe even two depending on the area, but if you are not producing allot of bud the electric bill will be to high to even make it worth it IMO. You could set the heater to come on once temps get under 60, but 65 is ideal as the lowest from what I am learning.
Adieu