Rusted Exterior Sewer Main Line, Replace?

[John Salgado]

I am in process of purchasing my first MF house and one of the notable items on the inspection report was the sewer main which is heavily rusted on the exterior.  The inspector was able to scrape off layers when taking a look shown by the photo with pieces on the floor.  He also noted some weeping below the pipe.  From my research, it could still be functional for a couple of years and the exterior condition may not be indicative of the inside.  I have no knowledge of how the current drains are performing, but what are some things I should look for?

• Would you replace before having tenants? (I am having a plumber to replace the main water line coming into the house).
• Or would you wait til its broken to address, which could be next year or in 10 years?