This AC costs $17000 in 2020 and assume will no problem for a long time but most expensive one and shortest time died. 

It will have even less effect on your networth or your net income,but it is almost risk free. 

Are there any red flags I should be aware of, such as the vacancy expense or NNN costs?

Size makes a big difference in cost.

This process can be very costly, requires zoning permission, in addition to all the items you listed.

Climate risks, cost, etc., are the other risks to gauge.If you live in Miami, I would def focus on Miami.

The idea behind this is it avoids having to get qualified by a bank and closing costs.

I have been having some success negotiating seller paid closing costs to help with rate buy down as well. - If you see a property that makes sense, I encourage you to get an offer in ASAP.

New construction may be an option, many builders are offering rate buy downs and closing cost assistance and then you get quicker appreciation when they continue to build out the community.

As long as you have proof of mailing you're okay on the time (not when she receives)  If you end up spending less to clean then you may have to refund some of the cost.