We are contemplating the same thing, what depth to put the sand. I have a 60 x 120 that is currently lawn grass. We are up on top of a hill so drainage is generally not a issue. We are thinking about 70 yards of sand, that would give us roughly 3" depth. Its a dirt "sand" (its not like beach sand) its finer and not so loose. My daughters instructor has it and it works pretty good.
Our only thing is we are thinking of dumping in on top of the grass due to climate issues. If we wait till its 60+ degrees to "round up" it first to kill the grass, we won't be able to get the dump trucks over to the ring. We would have to drive through the lawn and the 15 yd dump truck would get stuck or rip up the yard! If we wait till summer the trucks weight could damage our new black top driveway. So, we are thinking to get the sand early spring, when the ground is still slightly frozen and just covering the dormant grass. The weight of the sand should kill off a lot of it
My husband laughed and said the planets will have to all align to pull this off!