arduino and GRBL

Hi,
New to the forum and new to CNC world and not sure if this is the right place for my questions, but here we go.

I'll be building my very first DIY cnc. Nothing fancy, just hobby oriented. I'll be using Nema 23 servo (iSV57) with an Arduino. My questions are:

-do I have to install a GRBL on the Arduino? In other words, do I need a GRBL in order to build the CNC?
-do I need the MACH3 if I already have the GRBL? Do both accomplish the same?
-do I need a PSU for each Nema? Would it be overkill?
-for the Arduino, the USB provides the required 5V, correct?
-and just to confirm, an appropriate PSU for the motors should be one up to 48V, correct?

Thanks in advance.