I don’t expect (not do I want) every job and asume most owners are getting second opinions.However, I would not perform an estimate with another foundation repair company at the same house at the same time.  

What's the shape of the foundation?

I know foundation and roof are really expensive, I know floors and walls are perhaps less expensive.

I recommend you build a strong foundational understanding of real estate investing.1.

or a rule of thumb on how much money needs to be injected into a home to bring an outdated home to sparkling brand new (this wouldn't include foundation or structural issues)?

A person who likes interior design will think that is the MOST important aspect and not want to think too much about a foundation issue because it doesn't interest them.

This is your foundation, so you have to build that first, before any deals.

I love the strategy of small multifamily properties. 1 roof, 1 foundation, multiple tenants :)In the Charlotte market, this strategy is VERY challenging, just because the inventory is much more limited than single family/townhouse/condos.

Because you are in TX do you know how to spot foundation issues?

I suggest looking into - https://www.activedutypassiveincome.com/ to get a great base and foundation on it.