What do I enter as host name, and what as domain during installation?
The ideal configuration is that you have an own domain for your servers (e.g. myservers.tld) and number consecutively the host names (e.g. server1.myservers.tld, server2.myservers.tld, etc.). The domain myservers.tld should not be used for a virtual site (see here).
You can leave empty the host name and enter an IP address of the server as domain, but that would restrict the functionality of the DNS Manager.
In any case the host name/domain combination has to be reachable and has to point to the 42go ISP-Manager server.