If you get the high speed gearset then you can run any gears and aren't limited to Tamiya ones. Look at the Yeah Racing motor mount too, it has a slot instead of the set holes so you can set the gearmesh. This means you can run any combination of gears. I would switch to 48p while you're at it, they are just a lot easier to get than the mod .6

13.5T is generally a good match to Tamiya chassis too and the kit ESC shoukd run it fine. No need for the latest and greatest either, a cheap Trackstar motir would be fine, or look at the Surpass range, basically older ones are cheaper, later are better but more expensive, all work well