Not exactly sure of the metal but I'm thinking galvanized steel? I have yet to see any rust on any of my ladders .. You don't need the supports if you cut them in half which is about the right height for row covers bowed over into an arc. If you don't cut it, you will need supports.
I got large roll of the plastic at home depot almost 20 years ago - 16' x 100' - I had a house that was 3 feet off the ground and skirted it in winter. As long as it's kept in a cool dark place it won't degrade as I'm still using the original roll I bought. I used it to cover my 2nd greenhouse before I put the cover on it and this is the 2nd year I've used the same piece of plastic and I'm pretty sure I'll be using the same pieces next year as well .. no degradation at all in it that I can see.
