Do I need to buy additional software if I am only buying the Machine and 3D Printer attachment?

No, since UCCNC is included with the machine to run the G Code and the 3D Printer attachment comes with Repetier-Host slicing software.  Those two programs are all you need if you are just going to use your STEPCRAFT CNC for 3D printing.  If you are going to use the machine for cutting (spindle, laser, drag knife, etc.) then you need to have a CAM software program as well.