If your current version 8.0 is working to your satisfaction you should stick with it.
If you wish to upgrade, you should upgrade to the latest version which is v8.5a. The v8.6 that you mention is older than v8.5a (now does that make any sense from a version numbering point of view?).
We have the v8.5a upgrade CDs in stock at £14.95 including airmail delivery to anywhere in the world. There is only one v8.5a. It is the same all over the world and can be regarded as US/International, so would therefore be suitable for your purpose.