Florida homes are built on a slab so to install proper ac/heating you have to cut channels or venting channels in the concrete/floor you could come through the attic but that's not green and a professional would advise against it, is the ac your biggest desire to update?

When you change them, install the clean ones and take the dirty ones with you.  

You could even lay out some purlins and install real metal roofing.

The "resident" installed new doors that the owner paid for and sheet rock was also delivered.

I recently had a well known national flooring company come to my new house to install vinyl flooring in a bathroom.

I would install baby monitor cams in the common area so that he/she can see everything.

It was simple to install and very durable.

Some Future Maintenance / Prevention methods were included (but none of them listed fixing the Palmer Valve or installing a Sump Pump).

It is easy to install so you can do it yourself to save money.  

I always replace them for my rentals and also always install new ball type shut off valves.Good luck with your current projects.