Second, you want to settle the matter in their mind to keep them from seeking out other solutions.  

I do 't have a solution for your buyer getting a new AC unit, but I'm confused about something.  

I didn't have success trying to buy a house here and by looking for solutions I found Podcast #26 (MemphisInvest) on biggerpockets, I got really interested about what they do because I think buy and hold is a very smart strategy, especially for someone like me who knows how to live frugally. 

Check to see if they have an affiliate program.If you can get a feel for what investors need, then look for solutions to those problems.  

While you will definitely need to line up funding I think the harder part is finding reliable contractors that  you are comfortable with.  1. 

If that's not possible for the resident (I have some properties where that wouldn't work) then your solution of the lockbox could work.

There wasnt as many codes etc back then so it all boils down to how and who built it.I would suggest you have a reliable home inspector or builder look it over and discuss your renovations with them to get the best feedback.

The networking is beneficial as you grow and meet quality reliable vendors you can refer.Good Luck and let us know how it goes.

Are there any other creative solutions?

I'm trying to gain access to the MLS to get reliable information on recently sold properties.