It's most important to have a good understanding of G Code and machining techniques in order to determine if a post processor is setup they way you want. Joe Blows post for the same software and machine may be a total PITA for you. Every place has there own must haves in a post.
Get a good foundation, before you decide on CAM software, and a post.
If you are buying CAM software you should be able to get a decent post for the machine with out a problem.
"It's only funny until some one get's hurt, and then it's just hilarious!!" Mike Patton - Faith No More Ricochet