Pete, since Jim doesn't want something that big: I'm interested in your greenhouse. The gothic high tunnel that we are working on can't be used for potted material for a few years- NRCS rules. That means we are still very much needing a structure for potted material and seedlings. I'll send you a tree-mail.
Jim, the piedmont/gothic style of high tunnel can be attractive if it's tall enough. Are you hoping to have rigid covering or poly?