Should I buy the NCA Exam Prep one-by-one?

You should only purchase the Exam Prep subject you need guidance on as well as when you are planning to write the Challenge Exam(s). It is up to you as you are the best judge of what you are able to take on with studying as well as family, working etc.

It is recommended that you begin reviewing a course and see how you feel a week or two later and take on another subject if you feel like the material is manageable for you.