Foundation and Plumbing - Fix one or both??

[Miles Stanley]

So I have house under contract that I plan to fix and rent out.

Has foundation issues (center of slab is the 'high point" and as you move toward the perimeter, the slab drop increases...so the edges have sunken over time...its an older house).

Sewer test failed as well, cast iron pipes...typical of this area.

Debacle:  do I fix the slab, the plumbing, both or neither at this time?

Logic tells me to fix them both since the leak will eventually exacerbate the problem...not a matter of if, but when.

If I fix only the plumbing, the slab may react and worsen possibly since I am removing the source of the moisture that caused the issue.  If I fix the slab only, I see this repeating eventually.

Fixing neither seems like a mistake.

I am not afraid to handle the job, but I would need to get the house much more discounted than what I have it under contract for to make the numbers feasible.

Thoughts?

Thanks,