I am currently designing a caudron c460 and plan on using a very similar construction but it is my first design and I have been asking myself the same sort of questions. But I think I am going to use balsa as much as posible instead of ply and lite ply. The reason is for the smooth finnish and also the lite weight issue. I have heard plywood can be very heavy and also the strength is not as great as most people think for the weight also you only need strength in the parts that need be strong for when it is in the air. I plan on using the frame work out of balsa from the tail to the firewall with sheeting on the out side to behind the wing with spruce spars running full length on each side under the sheeting but still thinking of other ideas.