Starting my first ever 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. I have an oscillating 16" fan to move air around but thats it, in the building there's a small hole leading outside, i figured I could use that to my advantage and set up some sort of ventilation. But I have no experience in that area and have no clue what I need, and yes I have read all the stickies and threads and so forth, its just I get confused pretty easily reading different things all the time. So if someone could give me advice on what I would need to get proper ventilation going, and what I would need, I would be very grateful! thanks for reading! just tryin' to learn...

With an oscillating fan circulating the air I would think that it would vent out the chimney, should be sufficient, as far as heat in winter, that maybe a problem, the fireplace would help but how you gonna keep it stoked all night? passive heating would be your best bet, that is an electric radiator, I have one in my grow room and it works well, you can use HPS light from veg to bloom, it will work but less of a harvest, best have MH for veg and HPS for flower...just my opinion....best of luck.

Thanks for the reply! Well, I thought about a heating oscillating fan? bad idea orr? How about LED grow lights? Then I wouldn't have to worry as much about excessive heat, like from HID or HPS lights correct? Or would it be a better idea using HPS lights to counter the cold...

The temp in the room right now are like 70F, and humidity at around 33%RH whatever that means...is that ideal or bad?

I'd use the heat from the lights to your advantage, don't just vent it straight out the room. But that's just me.

My growing temps is usually around 65 to 75 deg, 75 being the ideal grow temp, I've used a heating oscilating fan many times and descovered that they are not as efficient as an elect radiator, passive heating is the best, your home is probably heated using radiators (passive heating), naturally MH and HPS would add heat during light cycle but then comes the night, as far as humidity I've never payed attention to it, never saw any problems with it, I'm sure there are some growers who have that problem, but fortunately not for me.