The 80 pound boat is going to be 1/4 inch plywod ALL the way - transom & frame included. If you got 3/8 or larger, the boat gets exponentially heavier and doesn't float any higher.
I just pushed this boat off a 9 ft high bracket - landed square on its nose from that height - oops! No damage.
That guy is 6-3 320-340 pounds adn the boat is all 1/4 inch with FG inside & out. That boat is also all of 7 years old.
Once the plans get there - read the instructions, watch the video, and then build your scaled version models. Now is not the time to rush and make a mistake.
The rumors are that poly absorbs water and epoxy doesn't. Epoxy breaks down with UV rays (sunlight) while poly doesn't. The list goes on. Both should be painted with a good quality paint and maintained. The boats should not be stored with water in them - the better you take care of them, the longer they last - epoxy and/or poly resin. At the least store it upside down on horses or blocks and repair any scratches or exposed wood.