I'm working with a company who owns a small, 19 unit, senior and low income apartment building.

I think your PITI estimate in both cases is low.  

Think of yourself of an asset manager, not a maintenance provider or lease-up specialist (but do those well too).3.

They are usually looking at C or D properties that local agents tell them will “cash flow“ They usually don’t cash flow because of high maintenance and vacancy.

Vacancy and maintenance factors should be higher than for Class A or B.

If you have more than one LLC, each LLC should have its own set of accounts.Checking: Collect all income here, then use it to pay bills, the mortgage, or maintenance.

That means weekly on going preventative maintenance fixing small problems before they become big problems. 

For example if you were to exit a new build in 5 years there may be no cap ex cost (i can think of few cap ex items that does not last 5 years - maybe carpet in class c and lower), just maintenance costs which would be fairly small.Best wishes

Your reserves should be part of the expense calculation: PITI, maintenance, capex, vacancy, property management, etc.

This vary heavily town to town. 20% down is great if buying strictly as investment but if you have a way of owner occupying I would explore that route and apply the 20% down to increasing value of property and instead using low money down loan.