Box steps

My customer wants to use 6 foot long by 2 inch thick bluestone for steps leading to front door landing/deck. He has mobility issues and wants each step only 5 inches in total height leading me to use 2x3 dimensional lumber as frame for box steps along with 1/2 inch plywood as top plate before bluestone is added. The bluestone as steps is non-negotiable. Is 2x3 lumber structurally sufficient for the weight of a 200# stone slab plus 250# bodyweight plus potential snow load? Spacing between frame will be of choosing -thinking every 10 inches. Is there any other material offered besides PT lumber you would recommend? PVC wood maybe? Or composite like Trex?