Making servo spin and able to detect angle
The way I'd do it is the following:


use an optical encoder with an endstop, they come @ 1€ on ebay, bought 4 today :-) - this will tell you the angular speed and incremental position
use a photo transistor to detect a specific point on the wheel (make a small hole or paint a spot in a different colour) - this will give you 
the absolute position of the wheel, to calibrate the previous step.


With this solution you do not even need a step motor, a DC motor will do it and probably has better characteristics, for the same cost.
So you could explore the DC option if it turns out that the stepper is not good enough.