Kite line winder prototype using 24V 250W electric motor (12V supplied during test) - full success :). About 3-4A of current drawn when pulling kite at moderate wind. Next priority is PWM speed controller and a fancy switch and cable box. Winder weights about 7 kg and was designed to fit into standard car trunk, although next version will be much smaller - as you can see there is a lot of free space on the frame.
I got inspiration from http://beachwinches.com page - a lot of great DIY winch designs.