For us in California the main metric is not "low cost".

I've done my fair share of research into good 'rules of thumb', but not any that consider the cost of land in the area.

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

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

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

Size makes a big difference in cost.

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.