DC motor and exact 1RPM (required for tracking) doesn't go together well without some sort of motor speed regulation.
In principle, the gear reduction could be anything, provided the speed of the motor is regulated to the required value.
For example, if you have a motor with 1000RPm, you need 1:1000 (more likely value fore DC motor) reduction to get 1RPM.
For this design, you do not need 1RPM - the arms of the barn door mount (shown at that website) and the driving screw can be tailored to accept wide range of motor rates.
DC motor speed regulator must use feedback (PLL, for example) - otherwise the rate will not be adequately precise.
I would suggest to you to try and find AC timing motor (used in washing machine timers for example). Their speed is determined by the mains frequency (50Hz, which is VERY accurate) an the output (after gearbox) is usually 1RPM.
Something like this (just make sure it is for 50Hz.. or you need to adjust the barn door lenght by shortening it for 20% to compensate for 60Hz motor runninh at50Hz):
http://cgi.ebay.com.au/Synchron-620-...item3360853bbc
http://cgi.ebay.com.au/MOTOR-1-REVOL...item483c6c632a
http://cgi.ebay.com.au/SYNCHRON-AC-T...item3cb16d53ce
Another (and much better, more flexible) option is /uProcessor controlled stepper motor. This way, you can have any rate you need.