Let me start out by giving you my opinion of Zentyal as a simple file server, knowing that your longer term goal is to add services. As of Zentyal 3.0, there really is no good way to think of Zentyal as a simple file server. You'll notice that Zentyal no longer has a file sharing only module. It's not designed to serve that purpose.
The result is that you very well could find yourself facing a setup that is more complicated that you expected. With file sharing now tied to domain services, Zentyal has become a little less friendly for NAS-type purposes.
Another result will be that not many of us are likely to be doing simple file sharing from a Zentyal 3 server, so our hand-holding skills for that purpose are likely not going to be too impressive. But, let's see if we can sort it out.
You're fine starting out with a single NIC. I would configure that as an internal interface, I think, since it doesn't connect to a WAN. Zentyal will be just another machine on your network for now.
What happens next I'm not sure about. Since you (presumably) want your file server to be easy to find, the simple way is to give the internal interface a static address that's consistent with your DHCP address scheme and server. You'll always know where to find your files with a static address. But if the file sharing/domain module depends on the DNS module (I'm not near my server so cannot look, but I think it does), then you have the option of referring to your file server by host name or address. Your choice.
So I suggest either starting from scratch and treating your single NIC as an internal interface during initial configuration or starting with that change to the existing configuration and see what Zentyal asks of you. We can sort out any complications as they arise.