You can chose just about any Bowden extruder for the Kossel Mini, I chose Airtripper’s Bowden Extruder.
The build is well detailed here on Airtripper’s site,:http://airtripper.com/1071/airtrippers-bowden-extruder-v3-updated-design/
Airtripper’s files are here: http://www.thingiverse.com/thing:35404
Note that you can find files that use a nut or the BSP fitting to hold in the tubing.