Linux CNC is the software that you will use to read the G-code file generated from Sheetcam and translate the code into a fully functional CNC operation. It will also be used to calibrate your CNC table along with jogging your tool into place.
SheetCam is the software that will be used to generate the cut path for your parts, this is where the G-code gets created.
DraftSight is one of many CAD softwares that you can use to create vector (DXF) files, or it can be used to edit already existing vector files you have. Although it takes a little time to learn, once you know how it works you can tackle almost any metal art project with ease.
AutoCAD is a great CAD software with amazing functionality as well as accuracy for more detailed and complex parts. With decades of profection AutoCAD has created a software that is virtually limitless with a proficient operatior. Although AutoCAD is on the expensive side compared to other free sketching softwares it is great for intricate designs. AutoCAD also has the ability to export DXF files allowing the operator to open them directly into SheetCam.
Remember, you can use any software that you prefer to create vector files. As long as the softwares can save or export a file in the DXF Format you will have no problems opening the file in sheetcam to start your cutting process.