If it held water before, it shouldn't need a liner now.
Check out local koi/pond clubs in your area when it comes time to restock w/the Koi. I have seen many people w/large koi they want to get rid of because they've outgrown there ponds. This could be a way for you to get sizeable fish at little to no cost. Many Koi owners will probally give you there fish knowing they are going to a big pond.
If you can, try to keep some of the catfish. They will be good to help cycle the new pond.