Hi DMF,
Welcome to the forum.
From experience, I suggest that an old PC (WinXP) with a parallel port but not having on-board graphics, which is dedicated to just running the machine with Mach3, may be more suitable.
Your laptop/netbook etc could then be used to run your CAD/CAM and create the GCode (I use a USB memory stick to transfer the GCode from my laptop to PC).
CAD/CAM software is really a matter of chioce but there are plenty, perhaps function limited 'try before you buy' to help you choose. No one software does it all so you will end up with a few different types depending on what you are planning on making with your machine.
There is a wealth of knowledge here on this forum and a wealth of members only too willing to help if you need assistance.
Good fortune with your project.
Tweakie.