Yes, about that heat. Here is what I've done with a 6x12. At first I thought to just keep the chill off. As it quickly filled with plants (close to 100) I ended up doing my best to simulate an intermediate environment. 55 to 60 deg winter nights and a bump to around 70 for daytime temp. We have had almost continuos low 30 deg nights for a couple of months now. Highs in the forties. Recently I removed the two 1500 watt (120v) heaters and installed a single 240v 2500 watt heater. Power consumption went from 25 amps to about 11 amps. Halved the power consumption and it is warm inside. With all the cloud cover and short days a 400 watt MH boosts the light inside.
It is nice that your greenhouse was a gift. You'll be spending your $$$ on plants!
