If there is moisture in the rudder, sooner or later, the rudder will fail. The steel armature that is inside a typical rudder (foam over steel with a fiberglass skin) rusts and fails, leaving you with no rudder and if that's while you are offshore--you have a situation.
There are some other posts on the topic of rudders, basically you can either ignore it (many do for many years) or attend to it by an extensive examination and rebuild, or a replacement. If you plan to go offshore--don't ignore it.
On the deck, you shouldn't need a steel plate if the deck is still solid. The only real question, is to get someone familiar with that to examine it and let you know what really has to be done. Deck structural repairs can be more expensive than you think, unless you can DIY. The web site for West Systems Epoxies has details about deck repairs, you might want to check that out to see what can be involved. I'd take care of that ASAP, since every day that goes by with more water in the deck, there will be more damage. If the leak is more than cosmetic--that can also mean the mast comes down and you've got an expensive problem. And another disabling situation. Tow boats are not cheap.