I'm rebuilding an old Ford Truck. One panel has rust on the bottom edge. My question is do I need to buy a new panel or can they weld a pice of metal in that area to patch it? As you can see I'm a rookie at all this... :o)

It can be fixed. Done all the time. If you are knowledgeable about welding sheet metal, you can do it easily. You need to compare the price for repairing that panel or replacing it with a new/used panel that has no damage. Salvage yard parts might be far less expensive in the end.