We used the Vex products, including the Single Reduction Clamping Gearbox - easy to put together, held up very well and overall very happy with the results.
You can get the Vex BOM and other info here:
http://www.vexrobotics.com/vexpro/examples-guides#VC
Here's a video of ours in action:
https://www.facebook.com/16737887669...6466343787954/