Cheapest way to go if you're going LiPo/Brushless-
TrueRC 8000 4S2P pack-$140
here
9XL or 8XL Feigao motor-$80
here
Mamba Max controller-$137
here
Castle Creations BEC-$22
here
Now, from this point there are 3 basic choices.
Direct drive with motor braking single speed only-
(Note, you will have to do
this mod as described by BrianG to eliminate the 2-speed)
RC-Monster motor mount-$60
here
A 5mm mod1 pinion-$12
here
Some sort of battery tray, rx box, and esc mounting area-$0-$50
Direct drive with mechanical brakes single or 2 speed-
Same as above minus the tranny mod
Clutch drive single or 2 speed-
Tekno conversion kit-$150
here
This may seem more expensive but it comes with a battery tray, ESC plate, rx box, and brake servo mount which you might buy for the above setups and the price will come out pretty close.
All these have their advantages and disadvantages so you'll have weigh them out and see what's right for you.
And if you want to go LiPo you will need a compatible charger and balancer.