Forget about powerful tanks unless you have lots of money or have your own machine shop. For small tanks you can use toy parts (or parts made for toy-sized tank robots easily found from websites places like Acroname, Solarbotics, or Nelwick Solutions), but anything larger with tracks that are actually effective and can be classified as powerful will cost you a few thousand ($4000 actually) at least, or you will have to become a very very good machinist. http://www.robotcombat.com/marketplace_machinelab.html
A nice small tank platform kit is: **broken link removed**
You can get separate tracks (plastic or rubber) and their corresponding gears and sprockets from www.acroname.com, www.solarbotics.com, or **broken link removed**
A good place to start is to buy a remote controlled toy car (a big one) and change out the body for your tank design. You can add weapons or other functions by using the transceiver and actuator parts from another RC car, and a second remote control.