I have noticed there are a lot more multi family units there that look affordable...

If you ever get a notice of a move out be ready to turnover that unit and get it move in ready as soon as you can to minimize the down time.I do understand the "house hacking" loan but 3 or 4 unit house would have decreased the pain of a vacancy. 

A common repeat question in this forum is "Where should I keep the cash I am saving for my first RE purchase?" 

Is it common, when makIng an offer on a commercIal multIplex, to place a contingency for all of the units to be under lease agreements?

And the 2 apartments I was shown didn't have any issues noticeable without actual doing a full inspection (e.g. baseboard heaters that don't work, and an unvented gas space heater).

One tenant of four on the lease moved in on Thursday and moved out Friday saying they did not feel safe in this Nashville  rental.The lease requires 30 day notice plus 3 months rent to terminate lease.

You could lose this thing as Jay mentions... this was far more common coming out of the deep dark recession.. but now a days builders don't need to do this and if they need to do this means they are either long on jobs or short on capital.   

Knocked on the door of the rental to let them know she was there, the dog was inside and started barking (which I know is common with dogs, doesn't mean they'll do anything). 

It was rough when I was there but noticed there is a Good Will there now and you can see some reconstruction going on currently.

I don't know your local tenant protection laws, but things like written notice given to vacate Unit B, a lease expiring within that window, etc, are typical.