I just am completing our 1st high tunnel. After hundreds of hours of research I chose Growers Solution because of the lower costs. I didn't buy their end-walls, they looked to be weak. I chose to frame in my own and am glad I did. I suggest if you have the funds to do so buying a better (heavier framed) house would be better. I had to put 2 center-post supports in to stop the frame from shaking even with the solid end-walls and still have over $10,000.00 invested. This included 150 feet of water lines, 2 hydrants, equipment rental and basic electrical as well. I purchased all the ventilation system and fans from ACF Greenhouse. There was a lot of extra's I had to purchase from the local hardware store (nuts, bolts, lags rope, etc,..). I used the Caterpillar Tunnel plans from Johnny's Seeds to set up the structure by as the manufacturers were basic and lacked enough information.
My conclusion is that poly or plastic pipe is best for seasonal covers and should be used only for year-round if supported right. Steel is a better choice. Johnny's Seeds has a pipe bender that makes the cost of smaller houses less, especially doing it yourself. Good luck and I'll answer any question you have.