Check out the threads on here in the build section for the guys building indoor koi ponds in their basements. Koiguy1969 has a basement pond he winters all of his koi in & he is in a similar USDA zone in Michigan to you. Check out his posts, only thing you will have different is that his pond is below ground & yours is more of a free standing building, like a shed/ cabin, correct? Winter heat would be your biggest issue, and I'm sure if you set up some kind of air ventilation like sissy's neighbor did, that could help your moisture problem , but I would expect that to not be a huge issue since the winter air is really dry. Possibly in fall/spring, but really wouldn't know until ou did it. First off, figure out your budget for th project & adapt your plan based off that. Good luck!