You actually technically are required to be UL listed, doesn't matter if you live in the middle of nowhere or if it's going on your property, FYI. And you really need an enginner to figure your footer properly/safely/to code. If you're not worried about an inspecor ever seeing or actually inspecting the sign and are doing this as an experiment, that's one thing, go ahead I guess, but don't try to sell a sign to the general public without going through the proper channels and procedures.
That said, for your first cabinet, I'd just buy a knockdown kit from someone and assembl it yourself. If you want to source the components individually, there alot of sign vendors who sell Signcomp extrusions, sheet aluminum and acrylic, and all of the electrial components you'll need.
For what to use on the structure/foundation (I assume you mean the pole and footer), that all depends on how big/high the sign is...