Neither Ubuntu 16.04 nor a Xeon processor are required.

"At the moment i'm writing all witnesses are using the version v0.19.2"

This is sort of true (in reported version numbers) but currently* witnesses should be building/running from the stable branch right now, because there has been a denial of service fix that is available on that branch and not the v0.19.2 release.
_{*This is subject to change, for potential future readers}

You briefly mention running screen but there's no explicit mention of keeping steemd running after logging out of the terminal.