I haven't been to this discussion strand for some time, so I apologize for answering today a request from early June! But it turns out I have two documents that deal with this exact question. One suggests two units as pilot units (or replacement units if you're using the AW text); the other suggests several units per grade level that are good starter units, for those in the process of gradual implementation. Since they would take up a lot of room here, I will email them directly to Cindy. But feel free to contact me if you, too, are interested in seeing them.
Megan Murray The Investigations Implementation Center TERC Cambrdge, MA Megan_Murray@terc.edu