WPL D12 upgrade - Brushless Motor, Speed Controller, X6A Transmitter and Receiver. 

You can put a battery as big as you like. I would put a big battery on the bed of the truck and i would do halls in the bed to pass the cables to the esc. Because is a custom work you will do halls and cuts and trimming with a dremel style tool. It's not difficult as it sounds. As long as you have the tools and the parts you can do a lot of things.

Hi, I use original 7.4v battery

@@jumpstar8064 what connector is used on the battery to connect to new esc?

For your reference: bulldog-army.com/electric-suit-waterproof-2430-5800kv-brushless-motor-25a-esc-set-for-1-18-1-16-radio-controlled-car/

I don't think the motor comes with the pinion. You should take out the pinion from the stock motor and put it in the new brushless motor.

The motor is 5800kv it has link in the description. The size of the motor is 2430 surpass hobby and the esc is 25 amp. When you open the link you will see it in the store.

Yes, the remote has changed to X6A Transmitter

Channel 3 or 4 also ok.

Yes

Should fit because it seems that the chassis is the same and the body is different.