Do not even take this project if it is GIVEN to you. As soon as you said it has been left uncovered out in the elements, i knew this was not going to end well. When you wrote, "the rear 1.5 metre of one hull is so water damaged that its virtually fallen off..." I started digging the grave. This is not a boat- it is a casket of dead dreams.
You really would be better off building your own cat rather than rebuilding this one- believe it or not, it will be less work, and you will feel more confident in the end result.