oil change at 3000 kms is very absurd, the old VW aircooled had this mileage interval and it used no filter, You are good for 5000 kms oil changes on this engine easily, if something like 3000 kms is written on the owners manual then ask Toyota WTH is it? This is a 2000 year above car not a VW designed in the 1940s.
You should be able to start a stone cold engine with one preglow cycle easily
You should ensure the engine reaches optimum temperature everytime the vehicle is driven
You should ensure the oil is hot before the engine is requested for power.
(meaning - if the car has direct fan and no thermostat - run away - its probably a pile of junk)
You should see blowby go down when the engine is revved to 1800 or 2000 rpm.
Visually verify the injection pump cold start device is working
audibly verify the injectors are not nailing
touch the high pressure injection pipes - you should feel the hammer effect in them, they should feel similar in all.
Ensure the timing belt is changed on time along with other service parts like water pump etc, make sure you dont see any rust in the cooling system or jugaarbaazi pipes etc.
Ask for when the brakes were serviced and what sort of parts were used, ask if they ever changed the brake fluid. Inspect the suspension and steering parts for good serviceability.
People who say "alhamdullilah I never spent a paisa on this car" is a sign of the car becoming a money pit for you. A car is a machine that needs tending and upkeep with spare parts. If they forgot this then all the jobs that were required under their ownership are now going to come out of your pocket.
the exhaust will be packed with carbon soot (its a diesel) it will be slightly oily too.