We haven't done Rev2Rev yet, but I have my guide here trying to work on our schedule for next fall, so I'll see If I can help you out. For History, Hearts and Hands, George Washington's World seem to be the main history books for the first 15 weeks. The Story of Napoleon starts in Unit 16, and The Story of the Great Republic starts in Unit 18. The Map Trek CD and US History Atlas are used from the beginning. The State Study book and Signers is used from the beginning as well if you are doing those. The Draw and Write Through History book starts in Unit 2. What in the World starts in Unit 6. The Thomas Jefferson CD starts in Unit 5. The Who is God Book and the Composer Studies start from the beginning. So in looking over the basic package, I think the only thing you could hold off on at the beginning would be the Abe Lincoln CD, The Story of the Great Republic (maybe), and The Growth of the British Empire. With the discount that you get for getting the set, I don't think you would save anything by doing it this way. It actually would probably end up costing you more. So maybe you can figure out some other way than trying to split up the basic package purchase.
For the Science you would need the Exploration Education and The Story of Inventions, Michael Faraday, and Four American Inventors. Henry Ford starts in Unit 19. Again, I don't know if it would be worth it to try to split those up, because most of the resources will be used early on and the package discount would be a much better deal.
For the Storytime titles you would need Amos Fortune, Songbird, The Secret Wish, John Paul Jones, The Reb and the Redcoats, and Marie's Home to take you through unit 16.
I hope that is helpful. So many of the resources in Rev2Rev are used throughout the whole year, that it makes it a difficult package to split into more than one purchase economically. I hope you can figure something out so you can purchase it all at once being able to save money and have everything you need.