Except as otherwise noted, all the books listed for this class at the Scholars Online Bookstore are required. A quick look at the schedule will, however, demonstrate that not all of them are needed by September. One does need to have the Horace in hand by midyear, but deferring the purchase a bit may diffuse some of the textbook sticker shock.
Both of the main textbooks are remarkably good. Ronnie Ancona's Horace has been available for several years, and it is a superior piece of work, loaded with helpful notes pitched accurately at the student working at this level. Her Catullus edition is equally solid.