Insulate the ceiling and the walls. Put a compacted crushed granite sub-base under the floor with a vapor barrier on top, and then pour a concrete floor. Don't know why anyone would want a wood floor since it is hard to maintain. A wood floor gathers splinters and bugs, not to mention dry rot over a period of years.
By the way, maybe you should have posted your question in the "General Discussions" category.
Good luck with your project. Sounds like you will have a neat storage unit when completed.
