Is our data in 2.4.3 compatible with 18.104.22.168 or will this require data migration of any kind?
If I remember correctly, it should work. However feel free to snapshot backup your existing 2.4.3.
We customised the login screen pages and some of the UI to reflect our branding, in an upgrade do you know if this is typically overwritten (these are xhtml files on the server we would have customised)?
There are couple of ways to migrated customization files there in new server. The easiest way is copy-paste.