Well...you have a few diverse questions here...
For some quick background, a stepper controller generates a very accurate sequence of pulses that drive a stepper motor via a power driver unit. While a DAC card can generate pulses, stepper motors tend to mis-behave and lose steps causing inaccurate positioning if you move them too slowly, or jump to step speeds suddenly without properly ramping the step sequence through a controlled profile. Now, I'm not saying that it cannot be done with a DAC board, it's just going to be a more difficult solution to implement in many cases. Not impossible mind you, but often frought with problems. If getting to a known position with great accuracy is of key importance to you, then ensuring that the correct number of steps is completed is a valuable capability in a stepper controller, all of the way to the point of actually using a motion feedback device, ie...an encoder, to ensure that you are where you need to be. The closed loop stepper controller boards support this capability direcly and provide it as an integrated function in LabVIEW.
With regard to any Serial or Parallel device, while LabVIEW provides powerful capability to use these ports, it would be up to you to define and implement any command sequences that a motion controller connected to this port required, unless a predefined set of LabVIEW drivers were built for this purpose.
Your PowerMac is most likely PCI Bus based, or if it's really old (like me...) then it could be NuBus based and you would have to check on the support for these legacy systems. PCI bus boards are very simple to use in PowerMacs...
If you go with a port based approach, you most likely will need to build a custom set of cables since I am not aware of any cables that would connect a serial port on a Mac to a serial or parallel bus based motion controller.
I would be curious, if you compare cost on a serial controller and drive/motor with an NI bus based controller and drive/motor is there is a significant cost differential...It is clear to me that the time and effort saved by using LabVIEW Motion Boards and VIs is a solution that "just works!" and this ease of use should make up for any reasonable cost difference.
If your project is to build a motion system solution from scratch, then go down the serial port path and possibly save money - at the expense of more time, but if your project is to use motion to achieve some set of measurement requirements, or implement a test system for larger purposes, then spend your effort on the real task at hand and use the motion control tools designed for LabVIEW...IMHO...
