Quick note: If you imbed the bottom board in the ground, as the ground freezes & expands, it will use the boards to lift the building & work the posts up some. When I lived in northern Minnesota we would either dig down below the frost line to do a footing or if using posts leave a 3-4" gap then lay rail road ties, rocks, sq bales, ..., along the outside to block the gap.
If you go with the steel roof (I would), for the supports it you screw a 2x4 flat across the vertical 2x creating a "T" it will greatly stiffen the vertical 2x & provide you a wider area to screw to. (Less misses)
Sometimes getting the neoprene washered screws to start into the steel can be a challenge. (Some have good points & others don"t) There is not shame in using a hammer or drilling a small (3/32") hole in the steel to get the screw started. DO NOT use the screws with a drill bit pointed tip. These are for screwing into steel purlins & will make too big a hole so the screws will not hold as well screwing into wood.
Have Fun!
John