The two requirements (reverse protection & reverse polarity) are not easily met in the same single device.   
By itself a MOSFET's Vgs is too poorly defined between parts and not sharp enough yto make a good low battery cutout.   
By adding the world's lowest cost IC and two or three resistors you can meet both needs. 

A P Channel MOSFET in the +ve lead, drain (not source) to B+, source (not drain) to Load+ and gate to B-, will provide FET which is turned on with correct battery polarity.

Now add a TLV431 shunt regulator from FET gate to ground and ise two resistors to program it to turn om when Vbat is above a preset limit. Add a 3rd resistor gate to B+ to turn the FET off when the TLV431 is off.