That is such an open ended question that it is hard to answer.
There are about a gazillion CAD software programs out there to pick from. The costs range from freeware (that are pretty capable) to spending $25K on something that will also butter your toast in the morning.
I won't make a suggestion on this, but I will offer this:
DO NOT get involved with BobCAD-CAM, period. Trust me on this one.
When it comes to CAM software for 2.5D stuff (including plasma) THERE IS NO EQUAL to SheetCAM!! It will cost you a few buck once, but Les is absolutely unbelievable. His product is awesome and he is fantastic when it comes to customer service and support. He will customize a post processor for your machine, whatever you need (if he doesn't already have it done) - and it will work.