They have bills to pay on the property like taxes, insurance, mortgage, utilities, maintenance and repairs, and replacing components of the home (cap ex).

Contractor still wants your business and agrees to original contract (often to his detriment) Supply chain issues + inflation = a bad deal for sub contractor. 

Did he actually rewire the house or just use existing wiring and bring plugs, switches, and visible components up to code?

All deals have two components that make them deals:1 - Lowest out of pocket (cash) you can do. 

Curious how you went about the management component.

With 1.2 you may have to opt for a bridge lender, provided there is value-add component that can push your DSCR higher.Of course, initial DSCR is inversely proportional to your purchase price: the higher the price, the lower is initial DSCR that is based on the current income.

You may wish to maximize contribution to a Solo401k first because you can elect to have a Roth component as part of your Solo401k.

As @Christopher B. pointed out there are many components to be considered, and quite frankly many more.That being said, "How do I get an actual cost estimate" and "where would one find an investor, if needed". 

Do you guys put separate systems in each unit or do you put one base station in one unit and connect all security components in and around the building to it?

For newly constructed, purchased or renovated properties and also retroactive generally over the last 10 years, building components are properly classified into individual units of property and accurate recovery periods for computing depreciation deductions.