Joe -- You need to go to more equipment auctions. You may be on a better plan with an old office trailer rather than a sea container, anyway. Even the twenty footers are really heavy. If you remember the Landoll trailer I had at Winchester? Did you notice the big Braden winch on it? That trailer was specifically set up for moving sea containers (20-footers as well as 40 footers). They are not trivial, even when empty, and require a hefty winch (and cable!).
What about an enclosed car hauler trailer? Even new, they are not horrifically expensive (by the time you get done writing cheques, it's probably close to a wash between a sea container, winch and whatever else you need, and just getting a new enclosed trailer). Putting a Maine tag on it is pretty reasonable, too.
And I can corroborate Brother Marlow's condensation claims. I had issues in my container, too (and mine even had a hole in the door caused by someone stabbing it with a forklift). I just left the door ajar and the problem went away.