When purchasing any house, old or new, you want to have it inspected by a qualified home inspector. They will look at the plumbing (anything visible) as well as the electrical system, roof, furnace, structure, etc. and let you know if they see any defects. If defects are found, you can negotiate with the seller for repairs. If something is just old and in working order (ex: furnace), you may have to accept it "as is" and budget for a replacement in the future.