Don't go too cheap on the plywood. ACX doesn't usually lie flat and could be
more difficult to work with. Marine would be nice or at least some premium
grade of exterior. The 1/2" would be lighter and adequate strength wise but
unless you filleted the insides of the corners , you will compromise the
strength. 3/4" will be easier to fasten the joints and have more leeway in
sanding the corners round to have a better look.
I would cover the whole exterior of the thing with glass and resin. And
reinforce the joints on the inside as well. Then fill the exterior with
either fairing compound or auto body filler, epoxy prime and then paint with
a good type paint (urethane maybe).But I don't know what your goals are for
appearance and weather resistance.
You may find some interesting ideas from the following websites:
www.classicseacraft.com
www.classicmako.com
and I think there is a Whaler site as well.
These guys are doing some great work on home projects like you are doing.

--
Ron White
My boatbuilding website is:
www.concentric.net/~knotreel