Which type of MOSFET
Rather than use discretes, I'd recommend to just use a digital inverter with 5 V tolerant inputs, but powered from the micro's 3.3 V supply. 

If this is the only circuit driving the RESET pin, you can use for example 74LVC1G04.

If you need to do wired-OR logic to allow other circuits (like a mechanical pushbutton) to pull RESET low, you can use 74LVC1G06. 

The cost of either of these devices is low enough that the placement of the part on the PCB is likely to cost more than the part itself (so that using a slightly pricier part to replace 2 or 3 cheaper parts pays for itself).

Edit:

I just noticed you wrote


  When the external pin is low, the RESET pin should be Gnd and if the external pin is high, the RESET pin should be disconnected (will get back to high because of the internal pull up).


For non-inverting logic with open-drain output, try 74LVC1G07.