Buyers must prioritize the engine condition and hydraulic system performance. Check for engine smoke color (blue or black smoke may indicate internal wear) and ensure the hydraulic pump operates smoothly without unusual noise. Key metrics include operating hours (ideally under 5,000-8,000 hours for mid-sized units), bucket capacity, and engine power (kW). Additionally, inspect the undercarriage wear percentage, specifically the track chains, rollers, and sprockets, as these represent significant future maintenance costs.