Some of the old software before 2010 may not be able to install/run normally on x64 or Windows 10 systems. Be aware of it before purchasing it.
Windows Server, version 20H2 is a Semi-Annual Channel release that omits the GUI for a smaller OS footprint. A new release comes out every six months and is supported for 18 months.
The difference between Windows Server 2019 and Windows Server, version 2004 is mainly how updates will be released. If you're on the Long-Term Servicing Channel, you're going to receive cumulative updates, similar to how Windows Server 2016 has (Windows Server 2016 coincided with version 1607).
Windows Server, version (x64) : Semi-Annual Channel
Windows Server: LTSC (Long-Term Servicing Channel)