John,
If you already know that you will NOT be taking the exam on/before 25-March-2018 then that means you need to study using materials that are based on PMBOK 6. Do NOT buy and use materials that are based on PMBOK 5 because you will be studying the WRONG material.
My recommendation: Buy PMBOK 6 and read it, while you wait for the Sixth Edition materials to become available.
Please be aware that there is a difference in cost for the new versions as described here
www.project-management-prepcast.com/price-change-announcement