You are in the right way. Keep in mind that you don't need to hack all the machines to feel prepared to exam. I know people that passed the exam having hacked 12 or 15 machines. A strong recommendation is to practice the BOF Extra Mile exercises of the pdf (1 and 2 is enough, 3 is a little sick and of a level not needed for the exam), and if you dominate this you'll have much gained in the exam (25 easy points). Also practice popular Windows privesc exploitations, also focus in exploitations based on web servers since this is an important point too