My local range i ssetup for 400 yards off a bench and also has a 60 yards "running deer" station and a clays area thats is for the public when the skeet and trap range isnt open. When I and the other members went to do the annual spring workday, we replaced the targets backs with 3/8" OSB or plywood. Most people wont be shooting slug guns past 100 yards so if you would like 3/4" to inch plywood works for bigger guns. I personally like 4'x4' backboards to give you the ability of placing up two or more targets at one distance. As for posts to hold the target backings, We sunk 4"x4" treated beams. They will last, but they cost a bit more than non-treated. You may want to make each distance progressivley higher than the previous one if you have level ground so you will be able to see every target. My range set the benches at a raised height by piling sand and fill up to about 5 feet higher than the ground level so you can see all the targets yet not effect trajectory. Generally we decide to make the shorter distances not as heavy duty becaus they mainly are used for handguns, patterning shotguns, and small bore guns like 22's. Use screws if you can so you can replace boards easy once they have been shot up. If you dont want to raise each distance higher than you can stagger them left to right and back to be ble to se them all. I would say you should need two to three bench stations so you and buddies can shoot together. Make it left hand capable if you might have leftys there. Well, if you have questions let me know.