Lidsey, when do you usually get your first frost? I have been told that 50 degrees is the time to bring them indoors, but, while we rarely get freezing temps., we do get close, and all of my "temp. tolerant" orchids stay out year round. My cousin who has the same temps, leaves all her orchids outside, and it doesn't seem to affect them. You could try bringing the warm growers in, and leave the others in a warmer micro climate until you get the greenhouse together. Orchids are adaptable, and unless you get a long period of freezing temps, the worst that will happen is that it will set them back for awhile. For example, the one I sent you has never been indoors, but since we had near freezing temps for three months this year, it had a puny spring bloom, but is now doing great. Of course we never had any sun until Aug.! By the way, the reason I divided that, and gave it away is that I still have the mother plant that is outgrowing it's basket, and a piece mounted on my lemon tree, which doesn't bloom well due to lack of sun.
If you really want to get creative, check the Oct. orchids forum on Daves garden. I know you can't post if you're not a member, but I think you can read what Maypoplaurel did with a couple of racks and some 6mil plastic, on todays post.