Don't worry we all started where you are.
Most home/hobby/light industrial conversions on these machines remove the existing old outdated and slow controller, and replace it with a computer with Mach3 installed as a controller.
This uses the parallel port as a machine controller, this uses a breakout board to interface the computer port to the existing drivers and relays. The spindle will need a specific board (0-10v spindle board) to interface the speed control from mach3