Novak HV-MAXX will be in your budget. You are limited to the max of 14 cells, unlike a non-sensored controller like Mike sales. the 6.5 will hit 40 mph from what I hear with 16.8 volts. You can also use stock traxxas pinion gears. That is another expense, pinion gears. The new 4.5 is faster I hear, but you will need really good batteries and they tend to get hotter.