If the existing steppers work, I'd be inclined to use them to begin with. Modern steppers will give a boost in performance, but as long as you fit good drivers along with a decent power supply, you'll still get reasonable performance from the original steppers.

Using an external motion controller allows for faster step outputs, and removes the step generation from the PC. Just using a PC, the step output speed is limited, and if you use Mach, the newest version of windows you can use is Win7 32bit (Win8, 10, or anything 64bit won't work).

With a stepper driven machine, you will get adequate performance with a parallel port and basic BOB, and you may not get that much in the way of performance benefit by using an external motion controller, however they do remove the problems of finding a suitable motherboard with a parallel port that will work well.