Hi,
I use Mach4 ($200) and an Ethernet SmoothStepper ($190) motion board. It will do all you require including slaving and axis squaring.
Others worth investigating are UCCNC and one of their motion boards like a UC300.
LinuxCNC with a Mesa motion board will work well too.
A more commercial one stop shop is Centroid Acorn. A little less flexible than the others but as near a turn key solution that you can get in this price bracket.
Craig