If you upgrade to metal tracks, I suggest you install a chassis brace kit, metal return roller support pylons, and install some ball bearings on the roadwheels. I run metal tracks on mine, and it makes it true heavy metal. The gearbox handles the metal just fine with no upgrades. It's the strongest stock design gearbox out there in 1/16. Another nice upgrade is more powerful motors. I haven't done it, but hear it is very worth while. AFV sells the brace kit, so does tank zone. AFV sells the upgraded rollers and support pylons.

Another cool thing is to install a mg. AFV sells these.

Other vendors would be etoarmor.com in Krimea, and Axels out of Germany.