The answer keys for the workbooks are not really needed, but you will need the test key. Most part of the tests consist of dictated words, sentences, and special sounds; you will not find them in the teacher guide but in the test key.
What I consider necessary is the Curriculum/Lesson Plans, the test key, and the Basic Phonics Charts. I have the old edition of the teacher guide and it includes a small copy of the phonics and seatwork charts, but I don't know if they are included in the new edition.
I would also recommend you the Basic Phonics Flashcards. This is optional but we have found them very helpful. We use them for daily review, making sentences, playing riddles, review games, etc.
Hope this helps!