G'day and Welcome
Decals are available at Maple Hunter, TM Tractor, C&G and any good CaseIH Dealer. Depends on which ones you want. They are not all that expensive.
I would recommend that you send the tank to the rad shop and have it Red Kote'd. When I did Ellie-Mae's tank ..
they cleaned and sealed the inside without stripping the outside .. so no dunking .. but the whole process is a lot better.
When I sent Granny's tank off to get done, they dipped it to inspect it to make sure there is no holes. Granny's required some welding for a bunch of pin holes in the bottom of the tank. You end up with a solid tank that will outlast us .. and not leak. It should be well primed on the outside when you pick it up.
I am happy with both methods, but having the whole tank done is way better.