Well CO² really needs to reach about 1800 ppm and then you hold it there for hours if you can. Then you exhaust the room and start all over again. Sure a fan can move the air around even if your room is sealed. It's not like you have a lot of air pressure building up. The plants make oxygen as a byproduct of the chlorophyll process so that is not a problem.

You could use tank CO² and save the heat. Gas heaters put out carbon dioxide so be careful with that.
For dirt farming make sure you have drainage of the runoff water from the buckets.

3 rooms
1 - mother/clone room
2 - Veg room 18/6 or 24/7 your choice
3 - flower always 12/12

An old mobile home is better choice (if you have not purchased the container yet) only because of the insulation and easy construction on the inside. FYI.