7.2 vs 8.4 vs 12 volt batteries

[Hotwheels]

Getting parts together for my King Hauler rebuild. Thanks to all who have advised and pointed me in the right direction with my truck. As some of you that I have talked to know I have be out of R?C for over 20 yrs. but I now have the truck bug. Here is what I have so far and trying to figure out the best route to go on the battery system for my hauler.
King Hauler
Box trailer
Stock Mabuchi Motor
Spektrum DX8 DSMX with AR8000 receiver
SMX Sound Module (on order Garden Trucking)
M20+ Speed Controller (Also on order from Garden Trucking
Original Tamiya Light and back up sounder(will remove when M20+ arrives
I have been reading post and threads on different set ups and am some what familiar with NIicad NIMH and Lipo. I have a general idea about volts(supply source) amps(capacity) and ohms (like a pathway for delivery of current).
My real question is how many volts should I run with the equipment going in to my Hauler. According to info I read about M20+ and SMX they operate within a range of 6 to 12 volts. The stuff I ran back in my racing days were Nicad 1300mah in stock class and 1700mah in modified class. That's about the extent of my battery knowledge besides cell matching our packs back then for peak charge/discharge rates etc. So I have a basic understanding. I was thinking of using a 7 cell nimh hump pack 8.4v 5100mah capacity. Is that to much voltage? Is 7.2v better. 12v don't think I need, but if you use them please share the benefit with me in doing so. Also since receiver is fed by main pack does increasing voltage above 7.2 also increase voltage to the receiver or is it regulated by speed control based on receiver demand? Just don't want to smoke any of these expensive electronics. Anyway share your thought's and I do appreciate all the great advise I have gotten from members on this site.
THANKS,
Lou