A PM sends someone for maintenance and that's valuable info on how the house is being treated.

My estimate is 90% of the time we obtain a tenant in our first open house.  

Since you'll be owner-occupying, take advantage of low down payment loans like FHA (3.5%), Conventional (5%), VA (0%), or a 203k loan for renovations.

Jacksonville has been building at record levels and is/was one of the highest cities in the country delivering new housing units as a percentage of existing housing units.

In addition to STR'ing the main house, he is building 2 more units (ADUs) on the back side of the property.

There are some amazing physician loans available that can allow you to purchase a owner occupied single family property with 0% down. 

It's a house sectioned into 3 apartments each a 3-bed 3-bath.

If you had not shared your post, as an owner, I would not be aware of the issues you are experiencing. 

@Bradley Buxton makes a very good point to make sure to do your due diligence as you won't be an owner occupant like your in-laws.Last bit of advice: Because this is a friendly deal, you want to take your time to make sure you structure it in a way that maximizes tax and cashflow benefits for both parties. 

In CA you can rent ADUs but can only rent JADUs as a separate unit if the owner lives on the property.