air cond is part of it (a/c compressor, a/c filter). check for radiator leaks, thermo and clutch fan in working order.
for engine do a compression and leakdown test before purchase. turn on the engine and check for blue or white smoke from the exhaust. get it up on a hoist and check undercarriage and chassis. rust is normal but too much is worrying. list of things to check is actually very exhaustive. but these are one of the main things id check.
if all else fails and u dont know what else to check for, talk to her and ask how shes been doin.
gl