I live in Northern NJ, not that different from your climate zone. My recommendation would be to attach the greenhouse to the house and built it as well as you can afford!
We/ Florian greenhouses
http://www.floriangreenhouse.c...
had our very sturdy greenhouse built on top of the garage, attached to the house. This was in 2001 and it is still perfect in many ways - It is connected to the heating system in the house ( oil heat) and on cold sunny Winter days acts as a heat sink, keeping the oil bill down. Air conditioning comes in handy in early Fall and sometimes in the Spring. I keep mostly Orchids and some C&S and move all my plants outside over the Summer.
I love walking in the middle of Winter barefoot in my sunny greenhouse ( glass ceiling) with a cup of coffee, bounce some gorgeous music of the glass walls and attend to my plants. Heaven!
