If you buy a ESC with the BEC built in you don't need to buy a separate BEC. You buy a separate BEC when you like fly large helis and planes and you want to have a separate battery for the RX so if you fly out all your motor battery you still have battery for the Rx(Receiver) and can land your heli or plane. There are other reasons also. But for a BeltCP or a Honey Bee King 2 I think a built in BEC will work fine for you. But you should have a 30 to 35 amp esc for the Belt cp just because there have been some problems running smaller ones and them catching fire.