Thanks! I think I found all the parts. Ordered!
For German GAF.
Teensy: http://www.watterott.com/de/Teensy-USB-Board-v20-ATMEGA32U4-mit-Pins
CP2102: http://www.amazon.de/gp/product/B00AFRXKFU/
Thanks for this, ordered the parts now as well.
Also there is another video with Driveclub and the Driving Force GT: https://www.youtube.com/watch?v=uzwcNtaNnjo