Kevin; The tanks are now painted on the outside with a "rust- proofing" paint, but I have'nt decided what to do on the inside. Any coatings for interiors are highly toxic, and I am considering just adding stabilizer to the fuel (diesel ) to keep moisture out as well as possible. This worked on my sailboat. Thy will be mounted on neoprene pads directly onto widened pads on the frames.
You have the rudder process correct. The mettal shop that I work with does all the forming for me and I do the welding , sometimes as in the case of the tig setup ,with their help. I get along great with the guys in the shop there, and the owner (3rd generation I've been dealing with !) lets me use most of his machinery ie 10 shear

.
As far as building in metal, I had considered aluminum ( found steel is simpler but requires good coating to which I am probably now sensitized to ) but was concerned about being able to prepare ane weld it correctly, so decided on the ply/wood once I found a low toxin epoxy. I am afterall a cabinetmaker!
FYI I've enclosed some pics of the steel sailboat ...a 34' Ted Brewer Kaiulani, that we built and sailed to the Bahamas and back in 2000/2001. Health and financial reasons caused us to sell it ( we really did'nt have the finances after finishing it to do what we built it for... the 5 yr loop around the Atlantid and Med. and Verna came down with rheumatoid arthritus quite severely)
It has been sailed to Europe by the new owners and has been sailing all around there for 7 seasons.
Doug