As far as your #5....I've used low tunnels for many years and all you need for supports is wire...pipes would be overkill and a lot of extra work and expense. If you buy a roll of 9 gauge wire and then cut it into the proper length pieces to span your growing area they will be the proper curve to just stick the ends in the ground without having to do any additional bending of them.
My project is to erect a cold frame that I bought used from someone. Must get the support posts in the ground before the ground freezes also.

I do you #9 wire for inside my high tunnel. However, outside, it is too windy to use just #9.

#5 for low tunnels use 1/2 sched40 pvc 10' lengths. 3/8 rebar 6 to 8" above the ground put pipe over it measure 5' over repeat rebar i put 3' apart by whatever length you want. i also cut pipe in 5' lengths to make row covers. 10' of pipe about $1 cheap and works great. john

#5 . . . have you seen all those political signs on the side of the road? After elections many are still left there. The signs are supported by heavy-duty wire that we use to make supports for Reemay. They are free and most people will gladly let you take them.

hmmm...maybe the wire I use is not #9. I think when i bought it (many years ago) it was a roll of brace wire for cattle fences. I can easily bend the wire with my hands. My low tunnels are just 2 or 3 feet high and I bury the edges of the row covers with soil. it gets very windy here too. this year i had a problem with a hail storm at the end of april when I had my tomatoes in these low tunnels. we had a storm that dumped small size hail a couple of inches deep and it collapsed a lot of the tunnels. First time that happened since i started using them (almost 20 years ago).