Best Server To Run Roon Core

I don't think you ever specified a budget.
If the rest of your system isn't $35k or above, you'd be hard pressed to do better on sound quality that the Innuous Zenith Mark III,  It's stunningly good and you can rip your entire CD collection with it.  It also handles USB audio output to your DAC, which IMO, is the best.

I use a fanless SD linux unit supplied in the UK by Inside Tech for £638. which has worked perfectly for a year with Qobuz (30,000 albums). 

Fanless Roon Rock Music Server - Intel i3 7th Gen 7100U

RAM4GB DDR4 (1x4GB) Storage128GB 

best input by far is ethernet.

you can see my system on Audiogon.

@newtoncr -- it was actually I who asked about the differences between Lumin U1 and Wolf Audio. Thank you for the detailed comparison. Very helpful, and sounds like you’re thrilled with the server. I was seriously considering Lumin U1 at one point but will definitely do some due diligence on Wolf Audio before making the final decision. Another product that I’ve great things about is Aurender N10/20, but I’m too committed to Roon at this point.

You can run a music server on any computer you want to start and sample the waters. 
You might find that to be just fine.

I never recommend going for “the best” right out of the gate.  The best may just appear to be best based on cost.   
Walk before you run. 
Also for cd res  streaming I’d recommend a wireless connection from server to streamer to help keep the gear that makes music isolated from what is typically a noisy wired computer network.  Others swear by using a wired network connection for the streamer.  Both done right will likely be just fine.