Scott Dattalo makes some excellent points in his response. I might
add a few comments:

>3) Beware of Logic level MOSFETs. They can be controlled with 0/5V gate
>   to source voltages. However, they have a fair amount of gate
>   capacitance.

Caution is required here.  The capacitance is largely a function of
die size.  However, the application doesn't require a large output
current, probably a couple of amps peak during  the energy
storage cycle, so a mosfet smaller than a TO220,  with a
less-than-ideal switching transition would probably be ok in this
application.

>there will be times when PIC is not active but the DC-DC converter is. (Like
>on power up, and software crashes).

Hamish mentioned he was using the '71 which doesn't have a PWM output.
If he has enough overhead in his code to provide a software PWM, than
it would most likely not function during software crashes or power-up.
I can imagine might be more of a concern if he used the hardware PWM
on the '74 for example.

                                      Mike