I use a propane heater for my greenhouse. I am in a zone 9a and have a lot of tropicals (zn 10-11-12 tropicals) that I have to overwinter in the gh. I am afraid to use an extension cord for electric heaters since we had one catch fire that way once when I was a kid (cord overheated).
I usually only run the heat in my gh if it hits 32 F at night. I do try to keep the gh closed up good during the winter so it will hold its heat and it gets good sun exposure during the day.

Sorry to hear about your cold temperatures! I have a fairly small greenhouse here in Indiana and I heat with natural gas. I got my 'furnaces' from Southern Burner and wouldn't trade them for anything. They can run on either natural gas or on a propane tank and require NO electricity. They do have a pilot light, but the thermostat kicks them on without electricity. There have been times when our electricity was out (we are rural) and I would have lost all my plants with any other heater. I have had two furnaces for 9 years and have had no maintenance issues. They resist rusting well. You can buy vented (requires a chiimney) or unvented, which might be fine since you wouldn't use it all the time. Be careful, though, with unvented...some plants like orchids can drop buds badly if you let them breathe too much exhaust!

As far as plant damage, my prediction is you won't have any. I regularly leave all kinds of tropical plants outside until ALMOST the first frost date. A few, like bananas, will show yellowing of the leaves, but only after a week of low temps. Most of my bromeliads will take right down to 32. I can't think of any tropical plants of mine that have ever been hurt with a single drop to 40 degrees.

Hope your plants are all fine...why don't you post in a week or two and let us know? Good luck with your choice of heaters. I would consider propane before I'd have a wood burner in my greenhouse, but that' s a personal choice.

Here is a link that might be useful: Southern Burner Greenhouses

If your house is warmer than 40 degrees at night I would bring the orchid and begonia inside.

Hi
I'm in 10 so much different situation but i don't even apply heat until the low 40's . Those in the yard are not even covered until the 30's there are a couple that spend the winter in the sunroom. All the plants you name can tolerate down to frost ,though the orchid might abort the flowers. The BIG difference in most tropical plants is frost .
Good luck gary

Thank you for the info Gary that does make me feel better. So far most things seem to be unscathed.

I don't start thinking about bringing anything inside until temps are going to go below 40.

"I am thinking a woodstove in the greenhouse might not be a terrible idea." That would make the air really dry & probably too hot. It should be cooler at night than during the daytime.

I wouldn't pay to heat a greenhouse up to 60 degrees. Heck, I don't pay to keep the house that warm some nights. I'd rather throw on some long johns & keep that cash in my pocket. Plants don't mind as long as they are not soggy wet, and never sitting in wet drip saucers.