If you're going to use the 2.5 trans I suggest you search the Traxxas forums for a mod that I believe was done by snoopmaxx. It involved making the trans forward only but the important part was using 2 gears on the intermediate and 2 gears on the output shaft, instead of just one on each. The gears are regular Traxxas gears and it strengthens the trans a lot which is needed for brushless use. Sorry I don't remember specifics let alone part numbers but this would be the cheapest option as well.

That Kershaw kit is meant for the grey transmission which is totally different from the 2.5 trans. The 3903/5/8 Emaxx trans is the best trans (basically bulletproof) but you need Blazerss' adapters to fit in a 2.5 Tmaxx or 3906 Emaxx chassis. The 3906 Emaxx trans will bolt to your chassis but they have weak points. I'm not sure if aftermarket idler gears and such are still available for the 3906 trans.