When sourcing a 400cc scooter, prioritize the engine type, specifically looking for liquid-cooled, 4-stroke, single-cylinder or twin-cylinder engines to ensure thermal stability. Key performance metrics include a power output of 25-30 kW and torque exceeding 30 Nm. Additionally, verify the presence of a Continuously Variable Transmission (CVT) for smooth acceleration and Electronic Fuel Injection (EFI) systems from reputable brands like Bosch or Delphi to ensure fuel efficiency and cold-start reliability.