Thank you for all the answers.
Now at least I know that the trouble of wiring the core will probably be easier than setting up LinuxMCE without a 2nd NIC in the core.
What all the linked articles and the replies do not answer is the conceptual necessity of a 2nd NIC. If you want to distinguish between internal and external LAN, there is no doubt about the necessity of two NICs. But if you setup your network as outlined below, why can't you just put your broadband-router into the internal LAN (with DHCP turned off)?http://wiki.linuxmce.org/images/3/38/After.PNG
If someone wants to access the internet directly from the internal LAN, it's fine by me
Or put differently:
In my home, the broadband-router and the remaining PCs are in the same room, but there is no chance to get an eithernet-cable to the core. With one NIC, I would just use the existing WLAN or a powerline-eithernet and I would be fine. But with two NICs I will be needing two distinct connections between the two rooms: broaband->core and core->PCs.
And I am getting curious about the conceptual reason for the decision to support two NICs only in LinuxMCE and make me go through the hassle of doing this setup.