heating and ventilation problem

hello, i have a little shed or place where temperature is almost same as outside and winter is coming so it gets outside maybe even -20c. But this is the only place i can grow, so i need to put some heating system into my box, which i will build over there. and i was wondering what would be the best option and the box is about 1.20m x 1.20m x 2.50m. Also is little problem about ventilation, because if the ventilator takes some fresh air outside the box, the air would be too cold i think, but maybe just little cracks in the box would make the air circulate better?

i'm doing pretty much the same thing, i've built a insulated, stealth box around my grow chamber - i'd highly recommend some insulation to cut down on heat needed