There is really no need for a receiver battery or an external BEC (battery eliminater circuit). All speed controls have a built in BEC capable of delivering 5 to 6 volts constant to the recreiver and steering servo. It does not matter what the input voltage is from the main pack. With a 2s, 3s, 4s, 5s and 6s battery, the speed control steps the voltage down to a constant voltage, 5 to 6 volts. Check the specs on the speed controls BEC if you want to know the voltage it will supply to the steering servo.
For e-buggy, high voltage servos are a waste. That is because the BEC is only supplying 6 volts. They will work, but you will not be maximizing their potential like you do with a 2s lipo receiver pack in nitro.
I agree with the recommendation to use a servo with at least 200 oz-in and a transit time under 0.15s at 6 volts.
Personally, I have a Hobbywing Xerun 150A speed control with a DS1015 steering servo in my Tekno converted D8 running on 4s.