She was a single mom in the past and did not build credit of her own. 

I've house hacked two single family homes in Reno NV, and rented out 4 rooms in each. 

If rent is ever late, you should have a policy that you deliver the notice to pay or quit and charge the late fee, according to the lease and the laws in your area.

Upon browsing Zillow, I noticed that there are some distressed properties (such as 3-bedroom single-family homes) that are priced reasonably between $60K - $90K.

I am interested in pursuing this strategy, but I don't have any experience with new construction, besides ADUs.My plan would be to build traditional 3 bed 2 bath single family homes with detached ADUs.

My lease is under my management company which is a single member LLC.. 

Secondly I am questioning the use of a business credit card to charge expenses of the rental property.

We are open to single and multi family, although we’d prefer multi of course.

I am starting to invest large sums of my net worth into real estate (SFR, or single family homes) in a city in California.

But you could also require extra cash for any repairs or upgrades that come up.Having reserves set up for unforeseen costs, vacancy periods, and ongoing maintenance charges is crucial, in addition to the down payment.