I used to sell Palram Greenhouses. They are not the best, along the line of most cheap greenhouses. They are NOT 4 season greenhouses no matter how they advertise them. 10mm double wall poly is decent, but still not that insulative. Expect to have a high energy bill if you plan to heat to 55 even in 7b. I had a similar greenhouse in zone 6B before I bought my new one, and I was paying over $100 a month for heating during the spring seed starting period, almost $200 the year I tried to grow all winter, and I was shooting for temps in the 40's, not 55.
There are no shortcuts when it comes to retaining/or adding heat. The better the greenhouse and its passive heating elements/abilities, the more you will pay for the greenhouse, but the less you will pay in heat in the long run.
My 2nd greenhouse I went with 16mm triple-wall polycarbonate glazing, a brick floor, a 425 gallon pond, and 25,000+ BTUs of Phase Change Material to boost the passive heating.
The greenhouse runs 15-20 degrees higher inside than the outside temps at night, but I still have to provide heat on nights when it falls below 20 degrees.
