First of all, a DIY snake is probably not going to properly clear 50 of main drain. That's a boy sent to do a man's job. You may be right about the condition of the drain, and it's a real possibility that the drain line must be replaced, but the first thing I would suggest to have a professional plumber auger and inspect the drain. There may be some clues about its condition that he would find when running the auger. The next thing would be to have a video inspection of the drain. That would tell you what you're up against. Drain line replacement is too expensive to do without knowing for certain that it is required. Compare it to major surgery, you wouldn't have open heart surgery without having a professional diagnosis.