Starting my first grow :)

Well I have recently decided to try growing my own marijuana, basically just because I need a hobby.
 I live way out in the country, and we have an outdoor sauna building that we never use, figured I'd try growing in there. The space is, 7' wide, 6' feet from the floor to the ceiling, and 9' feet long. I have 10 feminized,autoflowering seeds, soil, pots, just the basics. I now need lights, a ventilation system, things like that, and had a few questions! I'm hoping there's some nice people around, so far I've just got hate from other websites which are bullshit..
 Anyways, I was wondering if a 400w hps would work? Can I use a HPS light from seedling till its ready to harvest? Also, I live in southern Canada and it gets pretty cold this time of year, there is a heating option to the sauna though, it has a fireplace in the little entrance before going into the actual sauna, and that heats the inside of the building. would that be a good option, or would it like create humidity or some shit in the building? Inside, there is a small hole about 6 inches in diameter, and a chimney type deal that also leads outside, meant for the smoke from the fireplace. I'll upload pictures to give a better idea of what it all looks like. As far as ventilation, could someone give me a good idea of what i would need to ventilate the place, and keep the odor at a minimum?
 Also, the soil I have, it says it has enough nutrients or whatever to feed plants for up to 6 months? Do I still need to feed them?