Barry
The relay usually takes its power from the logic supply i.e. the battery connected to the receiver. Although it says 6v on the case it should pull in at a lower voltage. For example, the 8A relays we use have what are nominally 5v coils but they will operate down below 3v. I've never had cause to test a relay above its quoted nominal voltage, but I doubt if you'd want to use anything more than 4 x dry cells (=6v) for your Rx and servos anyway.
The power side of the circuit is where the
main battery voltage is applied and, as I said, that old transistor is supposed to be good for up to 60v.
If I were testing this out I'd use a 4.8v NiMH pack for the Rx or servo swinger, and a 12v gel-cell on the motor (probably a 360 or 385). Put a 10A fuse in the positive lead between the battery and the ESC, just to be on the safe side.
............and before anyone asks, I doubt very much if it has BEC* on board...................
FLJ
*Bothersome Extra Complication (seeing as how we aren't allowed to use even ersatz swear words now!)