The 16F627A isn't a good choice as it doesn't have an A/D converter. You can read a pot without one but it is more difficult.
Speed control on a stepper motor is done by changing the delay between each phase of the stepper motor drive.
What I would do is this:-
Each time TMRO rolls:-
Read A/D converter.
subtract result from accumulator
If accumulator has rolled, move on stepper motor one step.
Obviously there are several things that need setting up when it turns on, and the timer settings and the size of the accumulator need to be decided to get the speed correct.
You need power drivers to provide current to the stepper motor.
Stepper motors run much more smoothly if you turn off each phase in between running it forwards and backwards. The phase sequence I suggest is:-
A+ B+
A+ B0
A+ B-
A0 B-
A- B-
A- B0
A- B+
A0 B+
where A+ means energise A winding forwards, A0 means turn it off and A- means turn it on backwards.