As has been said before, it would be good to have a trusted mechanic with you when you inspect the vehicle. Do a road test and check for unusual noises, engine performance, smoke emissions, and puddles of fluid on the ground under the engine bay. Rev the engine to above 3,000 rpm and see if there is black smoke coming from the exhaust that doesn't go away after a few revs. If you can inspect the undercarriage, check for signs of rust or collision damage. Check for uneven tire wear and check the condition of the suspension. Check airconditioner cooling capacity --sa Carnival dapat kahit nasa 1 pa lang yung setting ng fan, malamig na sa cabin.
If the unit is casa maintained, ask to see the past preventive maintenance records to get an idea of how the unit was cared for. A typical PMS at Wheels Inc (change oil and oil filter) costs around P3,000+ if you use synthetic motor oil. The PMS should be done every 5,000-km.
Also inspect all electrical connections, function of all lights and lamps, wipers, washer fluid, fan belts, steering wheel play, clutch, etc.
Some suggested first maintenance items would be the replacement of the air filter, fuel filter and oil filter if these were not performed during the last PMS and if the condition of the vehicle indicates the need for such changes.