I don't have an answer for you, but if you can dismantle it far enough to access the metal parts, covering them with a thin layer of silicone should, I would think, slow down the rust.
Maybe look into cheap (so they're plastic) air compressor hose reels. Come to think of it, since you want all plastic, you're might be better off looking for cheap reels made with cheap parts instead of the good/expensive ones with metal.
One other thing, try and find parts diagrams for the ones you're thinking about buying. Looking at the replacement parts may give you an idea of how much metal is in it.