Cool project! I haven't looked at your book yet, but let me just throw out an idea anyway: Wolfram Language is sufficiently high-level that you'll start learning highly domain-specific functions very early on.
Or in other words: What you do just a few hours into your learning journey may already depend on what you're interested in creating. You might want to solve integrals, or create a game, or do comparative text analysis for history class - these different paths will diverge very quickly. 250 common beginner functions seems like a very large number to me - although 50 is reasonable, paired with an introduction to the Documentation Center.
Have you considered featuring a very incomplete selection of highly diverse example topics in such a way that students can extrapolate an approach to their personal field of interest from that?
(Disclaimer: I'm not a qualified educator.)