If you use a parallel interface and MACH3, then MACH3 is generating the signals. This is why you should run MACH3 standalone - no other software, no internet .... . Still there might be drivers for your particular hardware which decrease quality of signals because of too much interrupt handling code which runs unpredictably. There are chances that your machine will not run as smooth as it could up to it won't run at all.
For LinuxCNC there is a compatibility test which tells you wheather your hardware is good enough or not. I can't tell wheather there is a similar tool for MACH3.
I'd even prefere a controller solution if I would use something better than the netbook.
I bought my machine with SC USB board and the WinPC NC starter, as I was totally new to CNC more than a year ago. The maximum you can save is 20€ if you replace the SC USB board by the SC parallel board, but then you have no software at all.
I came along with the starter software for quite a while. But ... it's a pain not everybody will withstand that long, because the Starter at that time did not support automatic multi-pass cutting. So, if you want to cut something deeper than the cutter can handle in one pass, you have to cut once, update the cutting depth, cut again, update the cutting depth ....
A lot of people here in the forum use EstlCAM as a CAM tool, so I also bought this software. After several software versions EstlCAM additionally offered the possibility to directly control the SC with an Arduino UNO R3 board. This replaces the SC USB board and the WinPC NC software. Compared to WinPC NC Starter this is a progress, because EstlCAM creates/handles g-code, which gives you full 2,5D and even 3D cutting.
So, I gave it a try. I used the WinPC NC Starter to create PCBs for
1. an Arduino Shield, which can directly replace the SC USB board - it even fits into the machine
2. a parallel board, which is now fixed in my machine and connects via normal parallel printer cable to a PC or to the parallel 2 SC XC1 adapter
3. the parallel 2 SC XC1, which can be used to plug in either the SC USB module or the Arduino Shield
4. somewhen in future I'd like to build my own controller
Why? Because I like to have the opportunity. I tried EstlCAM and it is really nice for cutting all the 2,5D and 3D stuff I did so far, but it did not work nicely together with the PCB software.
The Arduino UNO R3 is flashed with fimware from EstlCAM. It accepts g-code which is generated by EstlCAM. The EstlCAM solution is also closed source, but it is inexpensive enough.
Of course there are other possibilities. For example grbl controllers (or projects dereived from grbl), which basically do the same thing. ( Just to add another piece of confusion
)
I think this was a nice trip on the road to "learning CNC", which did not cost too much money compared to the WinPC NC full version. 200€ versus 35€ + some material for the PCBs. And the doors are now wide open.