Is A Diode Affected By Amps Drawn From Power Source
If you're using a MOSFET for the power switch, then you don't need a "3 amp switch" to drive it, since the MOSFET's gate looks like a capacitor to the driving source. All you need is just enough current to charge up or discharge the gate capacitance in the time desired.

As far as the polarity of the power switch is concerned, you really have only two options, as shown below.


Use a common-source PMOS high-side switch, or
Use a common-source NMOS low side switch.


Of the two, the low side driver is a better choice because NMOS is available with lower \$R_{DS(ON)}\$ which translates to less power lost in the switch and lower temperature operation.  For example, an NXP PSMN1R030YLD will dissipate less than 4 watts with 80 amperes through the load.

R1 and R2 represent the RC receiver and the MOSFET driver, so they're shown hot all the time.