1st grow, need advice....

Planning to set up a grow box in the backyard shed. The shed is wood frame, 10 x 12 ft. The grow box will be insulated and about 3 x 3 by 5 ft high. I will be growing 2 or 3 autoflowering plants. My idea is to start the grow in Jan. Average low temp in this part of the world is 42F, over the 9 weeks of the grow the average low will get to 52F. I'm thinking I can use the heat of the light, and a thermostatically controlled vent to keep the temp in the 70-80F range. Does this sound like a viable plan? Also, for the space and small plants, is a 400hps overkill, or would a 250hps be better?
TIA

You definitely want to go with a 400w light for that space. My tent is the same size and I've got a 400w running and the is perfect for the space. As for your temps in the colder weather, you may run into some problems with the cold. But if you are insulating it, it may be fine. When you get it built, some pics would help.

think about a small space heater for when the lights go out.