If you're going to go with Tamiya products, probably their best option to get the speed you need would be their planetary gearbox; it has a ton of ratios, and you just add or subtract the units needed.
Most of time, for my projects, I just buy a gearmotor with the speed of gearbox I need - there's a ton of examples out there (new and surplus); the only drawback is that some of the "new" ones can be pretty pricey (I once busted a gear in the gearbox of a motor I had bought surplus - I contacted the manufacturer, and they said I could either purchase another motor/gearbox new from a distributor - for about $120.00 - or purchase the gear itself, but I would have to buy 50 pcs minimum - at $7.00 each! This was for a small gearmotor I had bought surplus for $10.00).
One of the other things to keep in mind for a gearbox is what the load is going to be; sometimes cheap "hobby" gearboxes of the Tamiya variety might not be up to the task, and you'll need to look into more expensive options (ie, metal gears).
Building your own gearbox should only be done if you have the means/skills to make fairly good precision assemblies; alignment of the gears can be fairly critical to keep noise and wear to a minimum (more noise and wear means less efficiency, higher current draw on the motor, more wear on the motor, etc). It could turn into an exercise in frustration, unless you need something really specific for the task (ie, you need something to fit somewhere an off-the-shelf solution won't go).