How much reliability would I gain replacing an old AC?

You can get a fantastic fan or another brand with a thermostat that you can have as a backup. It will automatically come on if the temp is high enough. This will also work even if the power was to go out for any reason so it will be better than just having another AC unit.

It is amazing how much cooler proper ventilation will do even in very hot temperatures. I would not want ventilation only to be what my pets endure on a regular basis but for safety I sure would like this to be the backup plan.