So, a couple of thoughts, from the "been there done that" department -
Get a 12 gauge extension cord, and get it as short as possible, so you don't leak amperage along the way. Putting it through foam pipe insulation helps also.
A heat pump may work as long as you are over about 30 degrees F, but after that you are just paying for normal heat anyway. And in the US most areas they have to be installed by a pro, permitted, etc.
What IS pretty cheap is a ventless wall furnace. 20,000 BTU ones, and a 5 gallon propane tank, will set you back less than $200. Only drawback is that they produce water vapor. I am in the high desert so that is great, but where you are, it may drive the humidity too high.
Good luck, and have fun with your new plant haven!
Carolyn