Will the PIC18F14K50 perform well as a CNC machine micro-processor?
This is a broad question, but yes, it seems you found a minimum micro that still does what you specified.

However, that may not be a good idea unless this is a volume product.  Stuff happens, and unforseen things come up.  You may very well wish you had a few more pins or a little more processing power as you get into the project.

I'll assume this is a personal one-off project (I told you in your meta question that this sort of thing is important context).  I'd get a 33F with lots of ROM and RAM for the main controller, and separate micros for each of the stepper motors.  It takes more than a single H bridge to control a stepper.

For firmware simplicity, get the same PIC for all four roles.  These things are cheap, but having to handle only one motor per PIC will make things easier.  The three motor driver PICs would run the same code.  They can receive commands from the main controller via a on-board IIC or CAN or something.  

The 33F series runs on 3.3 V, but the newer ones also have a lot of 5V-tolerant inputs.  The things you control inside your own hardware will be fine with 3.3 V.  The few things you need to control externally may be OK with 0 to 3.3 V levels.  Lots of things use "TTL compatible" input thresholds.  If not, add a 3.3 V to 5 V converter chip for the few signals that really require 4 V for a logic high.

500 ms makes no sense for stepper motor control.  You will probably re-evaluate the PWM outputs driving the stepper at 10 kHz or more, with PWM frequency at least 25 kHz.  That may sound fast, but 100 &micro;s (1/10 kHz) is actually a long time for such a micro to compute what duty cycle to emit next.  I'd probably aim for re-computing what to do to a stepper each PWM pulse running at 100 kHz or so.