You live a little bit further south than me, but it is safe to say we have the same climate. Storing planes in the building that you mentioned will be fine as long as moisture is not an issue. Plastic covering will expand/shrink throughout its lifetime no matter how it is stored. It can always be re-shrunk after a long storage period. As for the building, you can easily build a floor out of plywood on top of the floor kit that you can get with them. The best part is, if you look at a Sears or Home depot sale paper (usually in the local newspaper) you will most likely see a coupon for a free floor kit with the purchase of a building!