Even if you buy a bunch of rentals you have to stay very much engaged lest you lose your butt

You should meet as many players in the game as possible.

I regularly engage my previous CPA and need someone I can actually bring documents to physically when I have questions.  

Seller wants to engage in a owner financing agreement with the buyer.

However this company will also be engaged in real-estate sales and working with real estate investors to buy properties and then have our company manage them as rentals, or purchase to rehab and re sell. 

(Cash, Accrual, Mixed) Does the company have an accountant engaged?

I engaged a licensed HVAC contractor to do my central air systems- I’ve been burnt in the past, so my code going forward, is to buy materials myself, and pay the labor upon completions or milestones- The issue with HVAC is suppliers will only sell to licensed contractors, not homeowners.  

Single family houses would not work, for example, in most of Lower Manhattan.....Find your local meetings at MeetUp.com and NationalREIA.comLearn what investors in your area are buying and where the big players are making their money.Good luck!

If you are looking to make this officially separate it or make it a multi-family, I recommend engaging an architect in your area as well as the building department as you will need to meet certain criteria (independent entrance, another mailbox, etc.) 

Find some good players in your market and learn from them by serving them.