Any of the major linux distributions will do what you need. My preference is using something debian based because of the amount of software that is packaged in a convenient (.deb) form. Debian based includes Mepis (my favorite) and ubuntu and all the ubuntu derivatives such as Kubuntu. There is also Debian itself of course.
If you are interested in Enhanced Machine Controller --EMC-- downloading ubuntu directly from linuxcnc.org is the way to go. Getting this to work on another distribution is non-trivial. This disk can run in live/ram mode so nothing risked in giving it a try.
For cad/cam linux is not great. Here's a bit of what is available;
http://www.tech-edv.co.at/lunix/CADlinks.html
Qcad is easy and inexpensive, there is a free version available . The camexpert plug-in is deadly accurate for center-of-tool work in my experience. It is 2d only for cam and doesn't have many features. If using this be prepared to do much of your own code for cutter compensation etc. $40 for the plug-in.
Synergy is complex and powerful. $250 for 2 1/2 D (no simultaneous 3 axis movement) ~$1750 for full blown 3D. The cad portion is free.
Good luck with this. Linux is great but cad/cam is a weak point.