Jake
I'm pretty much at the same point you are on the frame, and my plan is to sandblast, shoot it with epoxy primer and a decent enamel. POR-15 or similar products (moisture curing urethanes) need topcoating for protection from UV, although the frame won't get that much exposure to sunlight.
My experience with powder coating, which would be real expensive on something as large as a frame (a BIG oven needed) is that any chips in the coating that let rust get a toehold results in the coating peeling off. Powder coating is also difficult to patch.
