Circuit Considerations
Your power supply concept makes sense (using a buck converter to make 5 V from 24 V), but your motor driver needs work.

The voltage follower you show in your second schematic won't deliver much voltage to the motor.  The FET will likely need a few volts G-S to turn on enough to allow useful motor current.  With the gate at 5 V, that leaves very little to the motor.  It also uses the 24 V supply very inefficiently.  Most of the power will go into heating the FET, not running the motor.

Your third schematic makes no sense at all.  A flipped N channel MOSFET has a internal diode from source to drain, so the motor will always be on with over 23 V on it, and regardless of what you do with the gate.

I would use a N channel MOSFET as a low side switch.  Connect the motor between its drain and the 24 V supply, and don't forget the Schottky flyback catch diode in reverse across the motor.  Use PWM to set the effective motor voltage.  For example, if the motor is supposed to run from 6 V, then use 25% duty cycle.  This is also much more efficient.

Something like the IRLML0030 can be driven directly from the 5 V PWM output of the microcontroller.  At 40 m&Omega;, it will only dissipate 40 mW with 1 A thru it.