Sounds like an excellent deal you got! I live less than a mile from Home Depot and Lowes (right across from each other) and am going to head over and take a look today. People keep asking me what metal to use for their board with the magnetic building system I discuss on my site. So far I haven't had a lot in the way of specific suggestions other than to go to a local metal dealer and see what they have.

My concern with your door is how well supported is it between the ribs? If it bows in then that's a problem. You may have to cut some sticks to create more ribs in the unsupported areas if needed. All in all sounds great. I love the size too. Are you planning to make the fixtures?