Mach 2 is as you describe. It will tie up the CPU.
You need a driver for each axis. have a look at
www.geckodrive.com
Unipolar or Bipolar would work with gecko drives
No need for encoders on steppers, and there is presently no software that supports closed loop systems. If you use servo's you will need encoders, but they go back to the drive, not the PC.
Build the machine first, purchase motors and drives last.
Eric