I have a similar situation in my outdoor grow box where I'm pulling air directly in from outdoors, down the the low 20s at night most of the winter. After trying several things, here's what I finally got that is working great:
http://www.qcsupply.com/commercial-...mount-variable-speed-controller-110-volt.html. It automatically speeds up or slows down my fan to maintain the set temperature. Now my temps stay between 77 and 82. Of course, my box is insulted so that helps a lot. I use a small space heater during lights out to keep the temps up then.
If you haven't seen this yet, give it a read. It will give you a better idea of what to expect from your plants.
https://www.autoflower.org/f44/life-cycle-auto-flowering-cannabis-5113.html