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**](http://www.onsemi.com/pub_link/Collateral/TLV431A-D.PDF) 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.
I nominated a 1.25V TLV431 rather than a 2.5V TL431 as the device cathode pulls down to Vref - 0.6V at best (based on my experiences and a logical result of the internal structure) and nominally to about Vref.
With 1.25V as Vref in the TLV431 and say 3 volts Vbat that leaves (3-1.25v) = 1.75V Vgs for the MOSFET, meaning it needs Vgsth of more like 1V to work well with 1.75V on the gate. MOSFETS that meet this spec are not common but do exist.
If desired an NChannel FET can be used but the level switching circuit will probably be slightly more complex.
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Minumum discharge voltage for a LiFePO4 cell umder loads of under C are usually in the 3.0 - 3.1V range. Under heavy loads. 1C 10C - ... lower voltages are usually due to voltage dropp from internal impedance. 3.2V is extremely conservative.
The circuit above drains the battery with R2 + R3 when the FET is off. So to minimise off mode drain R2 & R3 should be as large as possible while minmising Vset due to bias currents in the resistors.
A possibly suitable part - depends on desired max current [NTJS3151P](http://www.onsemi.com/pub_link/Collateral/NTJS3151P-D.PDF) - example only.