When sourcing a used Doosan excavator, the priority is the engine and hydraulic system. Check for smoke color (black or blue smoke indicates engine wear) and listen for abnormal noises in the hydraulic pump. Inspect the main valve and cylinders for oil leaks. It is essential to verify the operating hours; typically, a machine with under 5,000 hours is considered in its prime, while those over 10,000 hours require a deep discount and rigorous inspection of the undercarriage (rollers, idlers, and track links).