zoomkat:
Looking at the design paper it appears you will need an accurate tachometer that operates up to 4000 rpm, and a system to provide control to the driving motor to maintain the rpm under some changing load conditions. Not sure what of use for the design spec. can be made with only a $300 budget..
Basically for the speed what I did was From the motor I have it going to a speed sensor then to an ADC converter to the Arduino to the current amplifer and then to the voltage amplifer then to the motor again. The arduino's objective is for me to write code to have speed control for the user within a specific range. Then it change to that speed that the user implemented in decent speed. Thats why I kept saying "fastest possible".