You don't say which Mini Server you are interested in.
FWIW, I had purchased (way back) a 2011 Mini Server that I wanted to
use as a workstation. When I received it, it came with Lion and the seperate
Server app (first OS X version that did this). Once I was satisfied that the Mini
was fully functional, I booted to the recovery partition, wiped the OS X partition,
and did a re-install which resulted in a machine sans Server. There is no "functionality"
lost.
A little over a year ago, I took the same computer and and totally wiped both drives
and loaded Yosemite on it to repurpose it as a home media center.
So, the only thing "special" about Mini Servers is that they have two hard drives,
depending on year a different processor (in the case of the 2011 it was a quad core
i7 vs. dual core of the other models), and preinstalled Server app or OS X (Snow Leopard
and earlier). Other than that, it is just a computer like any other.
A server is just a computer. Some may have added hardware for communications and
other data handling (the Mini Servers do not). As a matter of fact, I took a basic 2010
Mac Mini and it has been repurposed as my home server.