CAD's easy, just do a search for free CAD, Progesoft has a great free Autocad like program, Alibre has something free I think.
Its the CAM that costs...
I'd just pick one you can afford, under 500 its probably Meshcam or Sheetcam, then if you have more for 500 I'd get VCarve. Its not exactly a CAM program, but in ways much more and has a lot of CAM features.
Next step up is OneCNC for around a grand, then BobCAM and some others around 2 grand, then the bigboys like MC and Gibbs at 2-5G or more.
In between theirs a gazillion others of course. Check out as many as you can. People like what they learn how to use. It all depends on how much you can spend. Do you have a budget? If your tight on money maybe Mach and the wizards will do all you need.