Your local megahardware store should have 12mm and 7mm pine ply that may be acceptable. They'll cut it to size for a very modest fee, leaving clean straight edges that are easier to work with. Do check the face for knots.
Soles really need something thicker. If I were starting again I'd go with 19mm or something equivalent, maybe even a hardwood ply if I could find it.
They'll have 90x45, 35x70 and 19x42 pine too, though you do need to pick the better pieces.
I found the better qualities are more expensive, limited in available thicknesses and quality, and quite often a fair sort of drive away. If you only need a small amount they should be fine.
As with all ply it's a good idea to actually check the thickness before telling them to cut away.
I've found so far that wood with a good coating of epoxy
resin a la WEST system is compatible with steel
in some situations. Best of all is to make sure it never gets wet...
As long as the metal remains dry interior
paint has not been critical for me. An enamel over a sound painted surface seems to be OK, and in the more critical situations I've used Penetrol CIP
with no major complaints so far. Results on the deck
and hull have not left me as happy.
I'm currently working through the POR15 system to coat the odd rust spot on deck
with a view to doing the hull if it works out ok, but it's way more work than just splashing a bit of paint on...