We ran a 300RPM 4in drivetrain, which has a good mix of torque and speed. It uses a 36:72 ratio from blue motors and gives you around 62.8in/sec and is around 29 holes long. If you are able to get blue cartridges later on, I would highly recommended it.
This is a great resource for drive gearings and making a drivetrain, I recommend you check it out: