Best Server To Run Roon Core

+1 on the SGC ST.  I run one without any storage.  Tidy maintenance free little package.
You could start out like many do and  run a Roon trial on a PC/Mac.  Then  Its easy to move the core to another machine.

+2 on the SGC STi5. I run one without any internal storage, keep my library on a Synology NAS, both outside the listening room. I use a Sonore opticalRendu as the renderer/network player taking the signal via ethernet and feeding my DAC. 
At $1K the Sonictransporter is basically a plug 'n play Roon core priced between an Intel NUC DIY build and the costlier Roon Nucleus(+). 
Since you asked, the rest of the system details are here: https://systems.audiogon.com/systems/133. Cheers,
Spencer

Everyone, thanks.  This is is all really good info.  For now, I would just try to run to one end point and expand in the future if I was to adopt roon.

I read today that in some cases Roon Core can be installed on Synology Nas as well. I assume this is NAS model specific and there are processing requirements that need to be met.

I believe you will find the NAS may be difficult to adequately spec to run Roon well. It will also depend on the size of your current digital library as to the level of server you will need to meet basic quality requirements for Roon.  Highly second a previous recommendation to look at Wolf Audio servers.  US company and they provide excellent information as well as after the sale support.  Reach out to Wolf Audio and speak with the designer/builder Joe Parvey about the different levels of servers available that could be custom built for you.