Many older steel frames seem pretty good for touring. I am especially partial to good lugged steel frames. I have 3, a mid-80s Bianchi Brava, an early 80s Schwinn Voyageur, and a mid 70s Trek. The Schwinn Voyageur was a bonafide loaded touring bike; it even came stock with 40 spoke wheels. Both it and the Trek have longer frames which provides a more comfortable ride because the frame can flex to absorb some shock. Look for extra clearance between the rear tire and the seat tube which indicates not only a longer frame, but also clearance for fenders. Be sure to look for threaded eyelets on the dropouts and stays for mounting racks for panniers.
Regards,
Raymond