You can buy the "standard" Reason upgrade to use with Balance+Essentials. This is the same upgrade you use for Reason 1-5. There is a different upgrade required for the retail version if Essentials.
However, because you can use the Balance as an "ignition key", you can either buy thew "download only" version direct from Propellerhead, or the boxed version from anybody (comes with usb key).
If you buy the download version, I think you'll upgrade directly to 6.5. The boxed version will include Reason 6, and you'll need to get the free upgrade from the Propellerhead website.