The air exit should always be at the highest point possible. Heat rises.
Hypothetically yes, but in a cold climate grow like this in which residual heat would be beneficial it's better to have a way to trap it (and humidity, as freezing temps freeze things like water) as well. One side vent with a reversible fan is probably all that needs imo- I'm saying this growing in the same climate.
Definitely agree with sealing the room.
OP, are the lights going to be LED's or HID? There's a substantial difference in the kind of electrical components you'll need for that size of grow depending on which one.
The heat from the LED drivers can be useful for a room that size, but it might need more heat than that in the winter. The power draw is much lower with LED's than HID's, plus the bulbs don't have to be replaced yearly (or ever) and aren't a fire hazard if touched with bare fingers.
If you're doing HID, you'll have to find a way to cool it during the summer but likely won't have to use supplemental heating in the winter. That might mean two vents across the room from each other, one for intake and one for exhaust.
You're likely going to need a humidifier, but can probably get away without a dehumidifier because ventilation under freezing temps effects both temp and humidity.
So your power draw is going to look something like 4 450 watt led/ lights, a humidifier, a bunch of fans, maybe a small heater. I haven't found AC to be necessary. Make sure things are wired for that plus more.