I'd suggest you start by looking at software packages. Look at Mach 3 - low cost, Centroid/Ajaz - mid cost, Camsoft - hi cost. (I'm a camsoft guy) Mach3 may meet your needs, but you do get what you pay for.
Do You have servos or steppers on your X,Y,Z axis? What sort of device do you have for the fourth axis? A few series 1 machines had tool changers, does yours?
If you do the work yourself, you should easily stay in your budget.