You need to ask the welder/metal worker to grind it down to a fine finish and tell them you'll leave it outside in the elements. They'll put a protective layer on it to prevent rust.
Any steel will work, just make sure you tell them what it'll be used for. If you are doing this yourself, look up the strength of the gauge you want to use and make sure you are taking into account more than just weight. The force of loading and unloading, etc.
Best of luck