But I would need an interent-ID to connect to the clients? I'm using the RU-server for the trial but would need to access clients outside of our corporate network if required.
That is all possible, of course. But if you enable "Generate Internet ID" your remote user will see that message anyway, although only once. This is necessary protection - Remote Utilities is a 100% clean and legitimate software. A program that skips the notification part is a rogue tool.
Is there a way to create a MSI/EXE which is truly silent, without any notifications (that it needs to generate an internet-ID or needs to send out an email etc)?
You can use the Standard or One-click options in the MSI Configurator, those options produce msi and exe files respectively. You can run the msi file then with a "silent" key.
Note, however, that the installation won't be completely silent if you enable "Generate Internet ID" (even if you don't select "Send via email"). This is a security precaution used to protect the remote user's privacy. The Internet ID is a firewall bypass feature meaning that anyone who knows the ID and password can connect to that computer even from outside the local network, provided there is Internet access.
If you do not enable Internet ID, the installation can be silent.
P.S. For anyone else to note, if you used address book sync in the past (i.e. in version 6.3.x.x and server 2.2 and earlier) and now have upgraded to the new version you must upgrade both Viewer and Server and then follow this address book sync upgrade guide. In a nutshell, you just need to export your existing address book, then set up address book sync anew - in the new version it is set up differently than before - and import your address book back into the system.