Depending on the specifics of the design 2 x 2 x 16ga could be used without issue.
When in doubt and weight is not an issue throw 11ga at it..
You can certainly powdercoat it.. (expect a couple hundred to do so though)..
And yes you can certainly do it yourself for under $100 in spray paint/bed liner..
Self etching primer to start followed by the finish of your choice RustOleum or whatever color you like.. Bed liner would be fine too.. I use that process for the rocksliders, bumpers, etc... that I've welded for my Jeeps and it takes a beating...
Any sprayed finish can easily take exposure to anything from this hobby..