The kit only provides the MDF and HDPE parts. All the aluminum extrusion, steppers, drives and so on you have to purchase. Generally a completed 4x4 is ~2500. The 4x4 is a paid design meaning you have to purchase the plans. After you purchase them though you have access to his private forums which from what I understand hold a wealth of knowledge.
The BOM for the 4x4 is in the plans as well as pretty much every question you are asking here is either in the plans or quickly answerable by Joe him self or via his forums. Every part you need is in the plans though. I believe he gives you the length of ballscrew you need but not a particular one as it's going to depend on what you want.