Since this is the 3rd time they are charging the unit with R-22, there is clearly an ongoing leak that needs to be remedied and likely won't be given the age of the unit and leak not fixable somewhere in the evaporator, and the first 2 attempts were unsuccessful.

After a few years of dealing with toilets, leaking this and that, renters not willing to change bulbs etc...

I would lean towards doing the roof and furnace repairs, then making sure you had reserves for replacement furnaces, which probably won't all quit at once.We are under contract on a house that needed a roof 15 years ago (no leaks but it is in awful shape) and it was a huge deal to get someone to come quote a roof replacement for me since it is for sale but I found someone who would.  

Bushings dry out and break etc.Things I look closely at when inspecting homes without power / water include:water damaged areas, look for former leaks, check everywherespend lots of time in the basement evaluating the joists and foundation.

(You can also get plumbing checked with an air test, where they put air under pressure into the lines and see if/howmuch leaking there is.

So long as your rental works correctly and has the basics (doors that lock correctly, outlets that have covers and work, sinks that don't leak, hot water that works, etc.) you'll probably pass with flying colors. 

There are no black spots or wet spots, and no leaks at all.

In the hallway approaching the unit I saw a fire sprinkler valve cover on the ceiling, at some point it had leaked and rust colored water stained the ceiling and walls, nice!   

This year there was a pipe leak under the foundation which the insurance wouldn't cover.In all the deal analysis I have seen, the insurance is always much cheaper.  

One of the units we managed flooded due to the unit above having a water leak.