When sourcing a used mini excavator, you must prioritize the engine condition and hydraulic system performance. Check for engine displacement and emission standards (e.g., Tier 4 or Stage V) to ensure compliance with local environmental laws. Inspect the hydraulic pump pressure and cycle times; any sluggishness or unusual noise indicates internal wear. Additionally, verify the undercarriage wear percentage, specifically the rubber tracks or steel links, sprockets, and rollers, as replacing these components can cost 10-20% of the machine's value.