Good luck. Nauru is not a common destination for people heading through the South Pacific since it is a very unattractive island because of the mining and the anchorage is not very good there. Also you need a pretty big boat to be able to accomodate 6 or 8 guests and their gear. A final thought, going to Banaba is pretty much exactly to windward, so even though it is not so far in terms of distance it would be a fairly rugged trip. Sounds like you might need a good-sized powerboat for this.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.