Marine Grade Plywood?
To me, it seems that "if you can''t afford the materials then you can''t afford to build this boat". and if this is true, how are you going to afford other marine items.
Are you going to use plastic cleats instead of metal ones? Are you going to use cotton line made for hanging your pants on instead of the proper cordage? Ete, etc.
Do some research then take the money you have and buy an existing, already built, brand name sailboat and go sailing.