In 1979, or maybe '80, my father and I built a .50 cal T/C Hawken. Took our time fitted the barrel to the channel, used a draw file to smooth the barrel flats, then browned the barrel, we both fired the gun when completed. Dad "put it on paper" and it returned a 2" 100 yard group with open "factory" sights. Many years later my younger brother borrowed that Hawken for an early deer season. He brought it to me the following May, worried. He had not cleaned the barrel since firing it twice in October the year before. The bore was very rusted, and he did not want to face dad with the gun in that shape. I was able to polish out the bore using "Cameo" (a product similar to "bar keepers friend") and HOT water, along with 0000 steel wool with "Ballistol" (a water soluable oil). That was 16 years ago, today I borrowed that same rifle from dad, and was able to shoot a 2" 100 yard group.
As for the missing parts...either locate a "parts gun" to salvage parts from, or walk away. The barrel can be saved, but it will take alot of work. If you do buy this one add me to the list that says check for a load already down bore. If it is loaded you really should pull the load before you do anything else. I recommend the compressed air method.