Engine hours, frame cracks and gearbox syncros — the three things that decide the price.
Ask for the engine hour meter, not the odometer. A truck that idled for two years in a queue shows low kilometres and high hours.
Look at the frame behind the cab and above the rear axle for cracks. A repaired crack is acceptable if it was drilled and welded properly; a painted-over crack is not.
Drive it cold. A gearbox that crunches into second when cold needs synchros, and that is an engine-out job on some models.