In order to create a 4th unit, it would be an extensive process that would require you to go through the zoning process (which would require you getting architectural plans, hiring a zoning attorney, getting land surveying, then going abutter and community meetings etc). 

Remember back to the GFC  big banks stopped construction projects mid stream and left high rises in various states of construction. you can still see a few in Vegas for instance..  

Many properties have high vacancy rates and need extensive renovation. 

Maybe requiring money be put in escrow to pay for specific improvements the buyer contracts with another party to perform (for instance , to put a road in).Thoughts?

OK, CA appraiser here, so I cannot speak to specific FL market nuances, but I have been doing it for decades and have extensive general experience. 

Make sure to take notes how to document your hours and types of activities.In the case Birdsong, doctor's wife demonstrated that she actively and extensively managed their rental properties and was therefore a “real estate professional”.

Note existing tenant when they modified contract and you did not agree means they have no extension.  

Then, perform extensive deep dives into each area—remotely, of course!

Buyer assumes violations and has 90 days to make repairs, with extension for weather related issues.   

There was an instance that the committee rejected our window carpentry due to craftsmanship.