The gearbox has a 10:1 ratio, which is part of a bigger project I'm working on.

12 VDC Motor, 60 RPM, High Torque
http://www.amazon.com/exec/obidos/ASIN/B00BX54O8A/donscollecti

Bearings, perfect size for 5mm bolt to go through.
http://www.amazon.com/exec/obidos/ASIN/B00F4M1B7A/donscollecti

8-20-15: Added STL of complete assembly to assist in seeing how the gears are positioned. There are a couple possibly combinations.

For the LEGO mounting kit, see thing:
http://www.thingiverse.com/thing:992083

The noise in the background is my printer running, it never stops!
http://youtu.be/ZrPO8_jl2lg
http://youtu.be/u9x0sFstHQE

PLA 20%, .2 layer, 2 shells

The M5 nuts, .1 layer, 3 bottom layers, 4 top layers, and 4 shells.

I also printed the M5 bolts for early prototype, The design takes two of the reduction gear set and tthree 10mm sleeves.

Hardware List

Home Depot
Partno, Description, Qty Needed
561-492, CAP SCREW 5mm-.8x40mm , 2 pieces
561-590, CAP SCREW 5MM-.8x50mm, 1 pieces
594-766, HEX NUT M5-.8mm , 5 pieces or print
366-757, FLAT WASHERS M5, 4 pieces

12 VDC Motor
http://www.amazon.com/exec/obidos/ASIN/B00BX54O8A/donscollecti

Assembly

I used a CAT6 connector to wire it up. The gear with the threads is off pitch by 15 degrees on purpose, it will thread onto M5 bolt fine, use a nut against the gear to secure it.

I included the drive coupler I'm using, which may not meet your projects needs, just adapt the other end to whatever you want.