The boats are quite tough, but each boat is different. If you manage to find one if a reasonable condition then you are a long way along the road to sailing one.
As you say they are ply and built in a similar way to the old Mirror Dinghy. They are not stitch and glue but make use of a frame to create the ribs and the plywood is then attached/ glued to the ribs. Some of the photographs on the web site will give you the idea.
The bilge keels are made from 10mm steel plate and can be fabricated by many yards.
The main advantage of joining the association is that there are a number of members who have restored/ currently restoring or are building new Lysanders.
The association has plans available for a small fee and these can be used to restore or build new.