With foundation issues, one could have plumbing, drywall, flooring and even roof issues and alot more.

Issues 1) even garage conversion ADUs in single family zoned areas typically cost more to build than the value that they add.  

If it has a good review history, there probably isn't any issues with the place.

There exists those of us that want a property in Costa Rica despite the issues we are discussing here.

Quote from @Robyn Henderson: My family's 3 unit prewar building in Brooklyn recently had plumbing issues and I'm debating renovating each of the bathrooms.

What's the most effective means of dealing with this issue?

Since you’re considering converting the upper unit into a short-term rental, you may also want to implement strict rules around smoking or strong odors in your house rules to avoid future issues.

The brother was fed up and needed to sell, but there was a significant issue.

What issues would there be if I put MY NC house in the LLC that has his name and my name on it?

While I think I might have paid slightly above the average price, I’m happy with the installation and impressed with how quickly the installers resolved the issue.