I have eliminated all of the obsolete Pan 6 items I could find from my business files and am ready to try converting. I can have no down time as I use Pan every single day of my life. I have OS 10.13. What version of Pan X do I buy to start and at what point do I upgrade Pan X?
Since you are using 10.13, there is nothing to stop you from running both Panorama 6 and Panorama X at the same time. You can literally run them side by side.
What you probably want to do is try opening your database in Panorama X and see how it goes. If everything works immediately, great, you’re good to go. If not, you can keep using Panorama 6 for your actual daily work while you get everything working in Panorama X. Once your Panorama X database is ready, simply export the most recent data from Panorama 6 and import it into the new Panorama X database. You can either do that via a text file, or you can re-convert the Panorama 6 database into X and then use the File>Import>Import Panorama Database into Current Database command.
There is no choice to make, there aren’t different versions of Panorama X.
The best time would have been a couple of years ago. The second best time would be right now.
The longer you wait the higher the risk of down time. At some point, you may find that Panorama 6 doesn’t work and then you’ll have an emergency on your hands. Right now, you still have the opportunity to make the transition in a non-emergency manner.