Designing Grow room.. need input.

In the complex i live, which is sort of like a multi-level condo I live on the top, with people living directly below me. On my back deck which is quite small, I have storage closet that is around 8x5x10ft high. The problem i have is that the breaker box is in this room, so it would have to be accessible at all times. This room is also "outside" and not heated, or in the summer cooled. There is a small wall mount heater inside it, that keeps it around 0 degrees Celsius to keep the furnace lines that run through it from freezing. My idea is to build, on the back half away from the breaker box, and cut the room in half. If i were to frame the outside of the walls inside the room with say 3 1/2 inch steel, insulate with something with a good r-value, and build it say 8ft tall rather then the full height of the room. I would end up with a grow room of about 5x5x8 give or take a foot each way. My main issue here is the temperature. I live in a region that gets VERY cold sometimes, -30 Celsius or colder and in the summer can reach +30. If i were to keep the outside space around 0 at all times in the winter, would It be hard to maintain a constant temperature on the inside of the small room? I'm thinking my exhaust fans would be all on the top, with one intake vent on the bottom near whatever i choose to use for a heater. Anyways, what I'm basically needing help with is deciding how this would be done, and how I can maintain a constant temperature even in the summer of around 20 degrees or whatever is optimal, somewhere around 78 Fahrenheit i believe. I am looking to have around 6 plants max in here. Thanks for reading. If you guys need a better visual to help me let me know and i can draw up some plans. Thanks again

get a good oil heater/radiator with a thermostat. Keep it set at the temps you want and it will click on when the temp drops below that.

Definitely an exhaust at the top and a passive intake near the bottom. Putting the cold air intake near the heater will cause it to think the temp of the room is too low and it will go on. Sometimes this leads to overheating as the heater never shuts off.

This can be remedied by a good exhaust fan near the ceiling. This will suck out hot air and draw cold air in. It might take a bit to get it dialed in but it is possible. Once you find the right balance of intake/exhaust and heater, you'll be all set.

Then you can start worrying about low humidity

Thanks for the input, yeah I have thought about the humidity problem already, that would only be a seasonal problem for the winter when its super dry. Do you think the same method would work for cooling in the summer as heating in the winter?

I think an AC unit would be required in the summer. You be pulling in hot air in addition to the heat from the lights. If you have AC, you might be able to run your intake into your living space.