You just keep modelling outside of the grid plane, you can put things miles away from it if you want. It isn’t a limit to how much space you can use (and you can change the size of it anyway, by going to View>>View properties) :yes:
Blender internal’s the one which comes with Blender, and Yafray’s an external renderer. I’ve only ever used blender internal so I don’t know what the differences in quality and time are.
Welcome to the forums btw