I am refurbishing a 30 ft Van der Stadt Black Soo, built in marine ply in 1966.  I has been in a shed for the last 10 years or so.  The ply and timber work is simply in perfect condition.  It is not covered in glass, but it is epoxied below the water line.  The topsides are finished in a single pack polyurethane, and no epoxy there, either.  Polyurethane on the decks.  Perhaps a good quality marine ply should not be overlooked if it is light and strong enough to use in the building of a Harryproa.  You are working with a technology that has been "mature" since the end of WW2.  I see a few blotchy and sick looking fibreglass yachts coming out of the water for time consuming attention.