Could be tricky to do without heat. Why chance it? If you don't want the trouble of propane, use a long extension cord and an electric heater. You can get electric heaters with thermostats on ebay for $~40, so it's not a big deal.

You'll want a good heat store in your greenhouse -- typically black-painted buckets of water. You'll want an insulated north wall. You may or may not want the side walls insulated. You'll want a double-layer thickness transparent south wall. Glass is really just for looks, but if that's what you care about, then spend the money. 90% of greenhouses use polyethylene, but polycarbonate and acrylic are good as well. For polyethylene and polycarbonate, get something that's UV treated so that it lasts longer. If you can get something that's treated to absorb IR, that's your best option, as it will help stop your greenhouse from radiating its energy away. Polyethylene is the cheapest; polycarbonate is the strongest; and acrylic is the most transparent and resistant to sunlight degradation.

Hi
I'm in zone 7 also, I have had a greenhouse for years and have always grown salad mixes in it. Many times all winter without any heat, I have a new 10x20 hoophouse we built ourself's I will use heating mat's for most of my spring starts and very little heat. We grow our salad mix in hanging baskets, just pick one and head for the house and dinner. We got a greenhouse tubing bender on ebay and built the new 10x20 for under $300.00