I hope that you have a large bank account!! The cold air of winter does not seep in from the ground. It is just like Daisy said, heat is your biggest expense. The more you layer the walls and the roof, the more money you save.
I had a greenhouse that was 300 square feet in zone 7 on Long Island. My floor was 4" of rolled builder's sand. it is extremely important to get it level. if you choose a greenhouse with 4' or 5' sidewalls, if you could dig it out to 2-3' deep and have it half way below ground, that will save you big $$$$$$. Ground/Dirt is a very good insulator!!
If you cover all of the remaining sides with Bubble wrap made for greenhouses like Daisy suggested, that will help. My greenhouse was double paned glass with a winter cover of 6mil plastic covering the walls and roof. The plastic was held away from the glass by using 1" x 2' lathe. I laid it on it's end creating an air space. This meant that it would hold heat a little better. AND IT STILL COST AN ARM AND A LEG to heat it!!! But that is the commitment you make.
You'll save even more shooting for 60 degrees then 65. better yet, 55 instead of 65. Every degree lower saves you money. That and you'll need Life Insurance when you LOOK AT THAT FIRST BILL in case of a heart attack.
In your case, it is so much better to overbuild something then to under build!!! Like Daisy, mine was benches along the outer walls with wood chips for humidity and I always had trouble keeping that humidity UP!!! The winter heating dried the greenhouse right out. I grew 98% orchids in mine like Daisy.
A very dear friend of mine grew VANDAS, tropical orchids form Southeast Asia. They {the Vandas} hated anything below 65 degrees! Hated it. She made the biggest sacrifice paying those winter electric bills. She used to say that she could hear cash register noise all night long as the heating costs mounted up. HER SOLUTION WAS TO MOVE TO CENTRAL FLORIDA! She saved a ton of money that way.
I am in no way trying to talk you out of this but telling you to carefully plan everything out. And remember that once you finish, a full greenhouse is cheaper to run one that is half empty. Oh, lastly, have a good back up plan for heat because at some point, it will be real cold one night and the power might go out. I used a supplemental heat for really cold nights in the form of a kerosene heater, running that baby on a low setting kept the green house at 62 degrees. Normally it was 55.
