Raspberry pi

For what would you like to use the Pi? CAD, CAM or as a machine controller?

As a machine controller (the part that generates the signals for the stepper motors) the Pi is totally out of scope - at least with the default OS. In all CNC setups that run without a dedicated mikrocontroller for generating the signals, you have the problem that the timings are not 100% accurate. This is caused by tons of interrupts and unpredictable scheduling of all the running tasks/processes.
You might have a chance with Pi 2 after putting a lot of effort in stripping down the OS.

But the mass-storage might still be a problem, because a SD card can have delays in a range of several 10-100 ms.

I plan to build my own CNC controller based on the Pi as well, but the signal generation will then be done by a mikrocontroller.

Have a look at the beaglebone black - there is a realtime liinux distribution called "machinekit" ( http://blog.machinekit.io/ ).

I haven't tried it personally, but I think this matches your idea the closest...

regards
Stonie