Lots of good information here but if I can make a suggestion Chris. Look at 6 mil UV rated plastic for your greenhouse.
We had built a plant shelter and just used 6 mil plastic from the builders supply. The southern sun makes the plastic shed like a snake skin.
When needing to replace it for the third time, I ordered 6 mil UV rated plastic instead and it was worth the cost. I didn't have to redo it at all and after 4 years when the bigger greenhouse was built it was still intact.
Hope to see photos of your progress...
