namelus, the two areas that you need to look into is the roof and the floor. The roof is flat so rain water may with time rust the corners so if you can build some small trusts so the rain goes down on one side that would help. If you do build some small trust on the roof you can add insulation.
On the floor you can put down 1×4 treated wood and in between the 1×4 a 1″ foam insulation then add your flooring on top(wood flooring).
There are many other ideas. This is just what I am thinking. Remember to put the container high and anchored well.