If your climate is anything like mine, one of your concerns is freezing your grow room.
I have no experience with this, s'all theory, but maybe I can still help anyways.
What you'll need is a 'backdraft' ventilation piece, so that when your fan is not running, no air may enter the grow room. You would have your fan attached to a thermostat, so that it would not overly freeze the room.
Depending on how cold it is, and how much much ventilation you have, this may or may not work.
If you're willing to pay a whole heck of a lot for heating.. then just have the fan always pulling air in, and set a heater on a thermostat in the grow area. This will create cold and hot spots in the room, so you'll need some fans to really stir the air up.
A combination of the two could work, too. Fan on thermostat pulls air in, until it's too cold. Heater heats up the room. Fan pulls air in again.
Let us know how it works