We've been waiting for an opportune time to get into real estate investing and have done a good amount of education and focused on building a solid foundation. 

Is he suggesting the grading issue is causing the foundation to shift, thus damaging either the supply line or drainage?

But, if you cover some foundational concepts of picking contractors, you should be able to get a pool of contractors qualified to bid your project, and from there make your pick based on price/gut feeling. 

Then the hardest part came; I had to cut a hole though the 4" Concrete foundation slab to access the plumbing underneath.

In some cases the foundation may need to be cracked open and repoured.

What I have found (at least here) is that many teams will be open to interviewing with you, it is up to YOU to choose the right fit.

Scale of work is ongoing but includes significant foundation repairs, new siding, new landscaping, as well as minor electrical and plumbing.

The land is in an older part of the city were mobile homes are visible but there is still some foundational homes in and around the area.

Investors looking to buy distressed property look for major issues (roofing, foundation, windows, HVAC, sub-flooring) and weight the cost of repair to the end goal (ARV) of the deal.

I look at my first deal as a way to get my feet wet, learn the ropes, and start creating a foundation from which to build a portfolio.