So I'm wondering what my best option would be for this extra cash.

Build in some extra income for yourself to take care of the things you know he can't take care of...

Honesty and the willingness to go the extra mile.

I personally negotiated a new deck, roof, and some other extras.

Effectively, it's like making extra payments on a mortgage, except you can then pull the money back out when, say, a credit card comes due.

Eventually, you might hit that number, especially with a lot of units in an area with really low vacancies and you're a marketing guru who can keep your vacancy rate below 3% (almost no one is), but initially, I'd expect it to be quite a bit higher, so it'll take a while before things are in the black.I think the standard number is 10% for vacancies alone and 5% for maintenance if you do your own PM, and an extra 3% if you don't. 

From my understanding it comes from the NOI but I guess you could bring in extra in the beginning to speed up the process?

The extra monthly increase can go toward down payment savings. 

It is possible to add the basement onto one of the other units to make the other unit have a den and extra bedrooms?  

There is nothing wrong with side hustles to get extra money but make sure you have verifiable income as well as banks will want to see this to get a loan.