I hear you, but my interest in a greenhouse is to allow for a 12 month option of growing plants and starting seeds.
For the record, I have worked in a commercial greenhouse (4 acres under glass/plastic), florist , doing cuttings, transplants, and retail and commercial sales.
If you are committed to a twelve month growing season ,
you are already married to it (grain farmer /dairy farmer), and have your priorities set.
Then take Cynicalbear's suggestion and look for one avialable for sale either on Craigslist, or some other reseller in your area.
Yes, I like to get some mileage out of the winter season too, but our patio doors let us do a few of those things. What I would like even more than a green house is a garden room that I can walk into through my patio doors, and grow lots of tomatoes, spinach, lettuce, herbs, and lemons without even having to go outdoors. LOL
One good crop for us is winter wheat. Plant it in October, it gives some nice green to look at in the winter, and harvest around the first of June which still allows for an additional crop to be planted and harvested. The large amount of stalks left make a really good contribution to compost.