You can mitigate some of the risk by:Using PEX tubing for the plumbing for anything you replace. 

My flip has a very similar issue to item 1 (caused by bad plumbing in this case) and the repair was quoted at half your price and included replacing a foundational beam as well.

We did a complete remodel including all plumbing and electrical.

As others have said hire and professional for this as a misstep could result in a larger more costly plumbing issue.

I bought two properties from them and both properties had foundation and plumbing issues.

It seemed like a good location, till I saw all the regulations... 

The age of the home is not as important as the age of the major components of the home, the roof, plumbing system, electrical system, age of paint and other building materials that may have lead or asbestos, variable expenses can creep up and kill an investment. 

Personally if I did everything possible to inspect the house and the plumbing I supply with the unit and its all good..... then I'm not paying a dime....not my problem

In most jurisdictions rentals of 30days or more aren’t regulated the way rentals of less than 30 days are.  

I need to know when was the plumbing, electrical, hvac etc last upgraded.