In most cases, you don't change anything on Windows Server. If there is AV running, you need exceptions for the SQL Server data, backup, log, and error log folders.

All memory management is performed inside SQL Server. You set a max memory there, and let SQL manage that. No need to change the Windows host OS.

I wouldn't install any resource intensive apps on the Windows host, but other than that, if you need it to function as a spare DNS or DHCP server, that can work.