Use this handy, dandy
greenhouse heater size calculator to determing what size heater you should use.
If you are going to use a little electric heater for heating, look into using a
ThermoCube. The fancier heaters will not turn on when the power is applied, they have to have a button pushed as well. The cheaper ones work better with the ThermoCube.
Something to help keep the greenhouse warm during the night, and even out the temperature spikes during the day, is thermal mass. Use this
thermal mass calculator to help you determine how much, and of what material, your thermal mass should be.
Get yourself a
remote temperature gauge. This link is to a mid-range gauge. The more fancy the features, and the greater the range, the higher the price.
Something you can do right now, with little or no money, is to eathier elevate the plants by placing them on a table (hot air rises, cold air sinks), or, keeping them in the pots, mulch heavily around the base of the pots.
Good luck with the greenhouse and enjoy.