Crushed Drain and Sewage Backup in Pending Flip

[Tim Porsche]

Hello All, 

I'm about to purchase a flip project that seems like a good deal, but I'd love to have your opinions to make sure I'm not potentially missing something. The numbers look good on paper...purchase price of $110,000, rehab all in about $40,000, and resale value should be around $220,000. 

Anyway, the house is up in the mountains and has a grandfathered in cesspit for sewage. I have had the cesspit load tested and visually inspected and it passed both tests. However, the downstairs bathroom has sewage coming up out of the toilet and all over the bathroom floor. That bathroom will be a full gut job. It's also backing up into the utility sink in the basement. The septic inspector said there is likely a crushed drain pipe between the house and the cesspit (cesspit is only about  10 feet from the house), and if this is repaired it should be fine. My contractor is estimating around $1,000 to replace that section of pipe. 

My question is, are there any other potential causes for this sewage backup, and anything else I should be thinking about that could make it much more costly than $1,000 or so to remediate? Right now the people living there are carrying water in and out of the house...obviously not a good situation.

Thanks in advance for any feedback and advice.