Reverse polarity protection on 5-12V input and output
A common rule of thumb for improved long-term reliability is to overrate electronic components by 100%.  Examples:  For a 12 V system, use a FET rated for 30 V or more.  For a peak load current of 3 A, use a FET rated for 6 A or more of continuous drain current.

Because your operating voltage range is relatively low (in FET terms), select a "logic-level" FET.  Check the datasheet, but they almost always are rated for 20 V Vgs, so you don't need a zener to protect the gate unless the reverse voltage might be way below -12 V.

A power MOSFET has an internal zener diode between the gate and source.  Pay close attention to the orientation of this diode in your circuit.

If your system can tolerate having the MOSFET switch in the GND rather than in the supply, an n-channel MOSFET will cost less and perform better.  Plus, the circuit is more simple.