This is not your only option, but I believe it to be viable. If you were to exhaust the room as described above, it would create a negative pressure atmosphere in the room and you could get away with doing just that! The fan will suck the air from the room outside, but also pull air in from other openings and crevices. It should work fine, but please note that when you start playing with active intake and active exhausts... the math gets kind of crazy. I prefer to have an active exhaust and not worry about an intake.

Sounds like you have a great environment to grow in!! With winter coming up, I wouldn't stress about heat TOO awfully much... I'm actually anticipating it so I can have more control!!!