I would say that the frame rails are the most important to me. If they aren't solid, they have to be either replace, capped, or repaired. In any solution you risk the chance that the car will become unsquare and eventually effect how well the car can be aligned and driven. Next, and almost equally important, for the same reasons, is the floor and roof stuctucture. This includes the front and rear pillars.
At that point you have to ask yourself what is the goal? Go back original, or just do what you want? If you are going back to original, the more complete the car the better.