get a experienced reputable Pro in to check the heat exchanger for cracks and other problems and be there to see it. don't call Joe's heat em and cheatem.
Temperature change: Measure room temp (intake to AC) at X degrees.Is there a way to measurably demonstrate the system is getting old and or needs replacement?
Assume they both are on their last legs and will nee to be replaced within one year. Budget for that work and adjust offer accordingly -or- insist that the owner replace them as part of the sale -subject to inspection-.Current owner says system was in when he purchased, so it's unknown how old it is.