Hi Charles,
Well, the Student Workbook is crucial for the students, because the worksheets there contain the patterns for the exercises the students practice. Those exercises will form good habits of mind for students, especially as they learn to think of something to say (i.e. to think).
The teacher materials you need depend on your experience as a teacher. Maybe it also depends on your courage. =)
The two parts for teachers are the Teacher's Guide and the lesson plans, i.e. Module Guides. The guides/lesson plans show you how to teach each day's concept - one concept per module. (That means three modules per essay/lesson - one for Invention, one for Arrangement, and one for Elocution.)
You could use the Teacher's Guide to obtain an overview. Then if you can present each worksheet's concept by preparing the student, giving him lots of examples, etc., you don't need the "lesson plans".
OR, you could buy the Lesson Plans and teach through the program without reading about the big picture first. That plan would be a bit like going on a hike without taking an aerial map. You could just follow the plan step by step, then look back later at the view once you've climbed higher.
It is nice though to see the big picture, then follow the step by step instructions.
(The CDs give the overview to the overview - kind of like "the shortcut to the shortcut" in Holiday Inn, but with a happier ending. ;))
Hope that helps!
Camille