Methinks you are in for an odessy of challenge. The general approach to using a PC for a controller is to use a cheap piece of hardware for a DEDICATED control function. It's only function in life is to take industry standard g-code and spin motors to make parts.
The use of a MAC as a controller does not seem to be a logical choice regardless of how comfortable you are with the machine.
The Simplestep product is a basic set of motion controllers. You will be dumped down into the depths of serial communications and using a proprietary piece of software to talk to their controllers. If you are lucky enough to get the Windows software to run on your MAC then all you will have to do is find/write an interface that will take simple g-code (which most CAM programs spit out) and translate that to their internal motion commands.
If you do find a software solution that takes g-code in and drives their cards (which in the quck scan I did don't seem to be motor "DRIVERS" but are the communications interface. They specifically exclude Gecko Drives (your best source of low cost power drives) The Sea-Gull solution is basically a CAM program that acts as the motion control too. You better like the way it works and the user interface because you won't be able to use anything else. It only imports HPGL plotter files (I'll leave you to discover the shortcomings of that approach!)
What you will be missing:
MACH3 Controller software. Over 7000 users and 5 years of development. Windows only. $159.00 for the software and it doesn't need "controller cards" running on RS- anything. Just a parallel port and a low cost breakout card and some good motor drives.
Xylotex 4 axis drive card. Directly drives steppers up to 2.5A. Takes Step & Dir in from a parallel port.
Gecko Drives for larger stepper motors or servos.
SheetCAM for 2D and 2.5D routing, milling and plasma cutting. 166.00 gets you a real GUI interface and things like tabs, parts duplication, different Posts for various controllers, etc
Vcarve for doing so amazing decorative and sign carving.
I could go on and on. I know that you are not in the US and that Windows based computers may not be as cheap as they are here but you will be jumping out of the mainstream of Small Shop CNC world in the US where most of the major help and activity is.
If your intent is to have a learning project then the MAC and SimpleStep will do a fine job of that but if you actually want to make parts rather than spending you time twisting in the wind by yourself then you need to look into what the bulk of this group uses.
Just my take on the subject.