I apologize, I am a bit unclear on your exact issue. Are you certain that it is asking you to upgrade the documents to the iWorks '09 format, or is it asking you to upgrade them because it is recognizing them as the iWorks '09 format? If it is the latter, you may find the following articles helpful:
iWork '09 works fine on Mavericks, I have both iWork '09 and the latest Pages, Numbers & Keynote on mine.
You do not need to install iWork '08 to get to iWork '09. They are two separate products.
You won't need Pages '08 because Pages '09 will open Pages '08 files perfectly, unlike Pages 5.
Amazon is selling the iWork '09 DVD boxed set for about US$27.