Welcome to CSC !! '73 Seafari 20' owner here - question I have for you, is which wood panels you are talking about - there are the cabin bulkheads on either side of the cabin door, and then there are the cockpit side panels - which may or may not be covered with foam / vinyl coaming bolsters.
The side panels could be done in plywood, probably 1/4" would be fine, and coated with a polyurethane varnish, or painted - that's probably the quickest, easiest solution - or get a boat shop upholstery supplier to redo the coaming bolsters on a plywood base.
The cockpit pulkheads are structural, and would need a thicker plywood, probably 3/4", again could be painted or polyurethane varnish - the originals on mine are covered with a wood grain melamine / formica material on the outside (cockpit side) and covered with the "cabin fur" on the inside.
And yes, the boat is definitely worthwhile - this is a 20 footer that can go where very few if any other 20's belong!!
I assume it looks something like this -
Good luck with her, and feel free to share some pix !
Bill