The two prerequisites to introducing the first 2019 domain controller are that domain functional level needs to be 2008 or higher
https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels
and older sysvol FRS replication needs to have been migrated to DFSR
https://techcommunity.microsoft.com/t5/Storage-at-Microsoft/Streamlined-Migration-of-FRS-to-DFSR-SYSVOL/ba-p/425405
I'd use dcdiag / repadmin tools to verify health correcting all errors found before starting. Then stand up the new 2019, patch it fully, license it, join existing domain, add active directory domain services, promote it also making it a GC (recommended), transfer FSMO roles over (optional), transfer pdc emulator role (optional), use dcdiag / repadmin tools to again verify health, when all is good you can decommission / demote old one and move on to next one.
Any currently supported operating systems for member servers and desktops are fine to use.
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