Thank you @flmaxey for your reply.
My main need is a server. The idea is to run OMV on a Raspberry Pi which I put in the closet and that's it.
However, I need eiskaltdcpp/ which I mentioned before.
What is unclear for me is how I can install something that is a desktop software in an environment like OMV. Once installed, I will use it as a server software and access it only from a WebUI.
Since you explained that installing OMV after Debian would make it a less good server solution, I rather have OMV first. But can I still install desktop software on OMV?
Out of curiosity; if I want to install OMV after Debian, shall I use the same installation ISO or another one made specially to be installed in top of Debian?