I won't argue the vent/no vent case. I chose a vented Southern Burner A-1 rated at 25,000 btu and I need that much. My greenhouse is a different shape, it has about 1200 cubic ft which should be you approximate volume. We get down into the 30s quite often and sometimes into the 20s. I have 4 fans going , one of which moves air down so I get a pretty thorough mixture of hot and cold. You might be grateful for the extra capacity, and the Southern Burner comes with a thermostat that doesn't need an electric feed to work.

I started my violas in December by putting the seeded "pots" on top of the steam radiator covers in my house. Perhaps you can start the seeds inside first, then move out to the hoophouse once they're germinated. I did the same thing last year for my impatiens (not NG): started on top of radiator covers in March, and they looked great by mid May. The impatiens were put on South facing windows while staying inside. I'm finding the violas need to be under a CPF lightbulb to flourish, but I give them some sunlight when available. Hope the picture shows up:

The HAF fans are hooked up to a speed controller set in the wall of the house out there. It is not connected to the greenhouse controller. They stay on 24/7 during the cold months to keep air moving inside so there are no cool spots or areas higher in humidity than elsewhere. When the temp really drops, I set them on high. The power window is set to open first if temps rise. If temp continues to rise, then the first roof vent/fan kicks on. This allows for fresh air to be drawn in. A second fan then kicks on if needed. The fogger drops the temps as well. Here's some shots of the electrical stuff. This first pic is where I got things set up for my electrician friend. Shot of the relays. I would never have figured that crap out! This is the Bartlett controller. The two green bundles hanging below are for the thermistors. The gray one to the right is for the humidistat. I didn't get a shot of the power window controller we attached to the side of the relay box. That even gave my buddy a headache getting figured out! I finally got a shot of the fogger in action. Fog on fog off! The first time this thing ever kicked on like this I had all my tools and things scattered all over the greenhouse. I'm screaming at my wife to come help me get them in before everything gets wet. Within a few moments, we couldn't see a foot in front of our face! Today was very dry out and it was kicking on a lot.

Your friend should go to the local propane dealer. Somebody like Suburban, Ameri Gas, Indiana Propane. Or a Recreational Vehicle supply store. I am not familiar with the stove. He will need a pressure regulator, hose. Tank should be outside. There is a multiple tank hook up. those items may cost $150-$200.