I believe x7 stops (officially) with Win 8.1. Not to say that it won't work on Win 10, one of the nice things about the legacy bloat of Win 10 is the ability to run older programs.
x8, I believe is the oldest that you can go with "official" Win 10 support. Although considering both are legacy versions, I doubt you'll get much "official" support either way.
Normally, I would say if one is upgrading now, might as well have the absolute latest, but I do believe 2019 has it's issues from what's being reported on here. Both it seems coding wise as well as ancillary "concerns" as well. I would get the latest that appears to be the least troublesome.
One thing to keep in mind though, due to the rolling release nature of Win 10, any new(er) update could cause issues with those legacy software and it may purge it from the system without user input (my dad had updates twice on Win 10 that left a message of programs not compatible with this version of Win 10, so were removed). Just fair warning on that. Something to think about.