If anyone is still looking at this, I face similar challenges. I grow in an outdoor shed. Its 20 degrees low at night. Using a 600 w. HPS light I manage to keep the temps at 60 to 75 degrees on those cold nights. Light stays on 24/7. I have closed off part of the shed by building one 4x5 section with plywood and a dark room 4x5 which I use the below heater in. This heater cranks,,,,80 degrees easily. The thermostat is tricky on this heater though. I recommend the garage grower build a room in the garage or buy a tent . CFL's wont help cold prob though. If you build a room, go at least 4x5 and 6 feet tall. call it your tool shed if anyone asks.
Vornado VH110 Whole Room Vortex Heater