Chances are your only real choice will be A36 which is the most common hot rolled steel. Its inexpensive relative to the other steels and will be plenty strong for this application. If you have a local pick yard you may be able to find most of the steel you need for less than 1/2 the cost of retail. Our local yard here is 36 cents a lb for all steel over 100lbs. There are charts on the net that will give you the weights of different steels based on length/diameter/thickness to give you a ballpark as to how much you will spend. I am not too fond of the design, mainly the center rotator tube appears too small for my liking, I used a 3.5" sch 40 tube on mine. The center is where the load is distributed and I want overkill there...