I need some information about redoing a shower and shower pan?

If you install a liner specifically made for showers and then pack a concrete base over it, you should have a shower that will never leak again. The liner needs to go in before anything else, then the base, and then the backer board for the tile or stone. The liner extends up the walls so there's no chance of a pan leak. If that's what the contractors are telling you, they are correct.