I assume you want something pre configured with an OS and Roon Core already otherwise you are really justing asking if someone commercially sells computers.
DIY. Is pretty much your only choice. Specifically because coax s/pdif has been phased out as a normal computer output. I had to use a special header off the motherboard to get S/PDIF on my last build.
Are size, aesthetics, noise and price also a consideration?
As Daniel said, it seems many motherboards no longer have an SPDIF output connector as standard although many do have a header to which you can attach an SPDIF output connector that is often included with motherboards or can be purchased separately for a quid. Failing that, buy a PC that meets your needs and then add a sound card with an SPDIF output. I felt that the sound from the output from the SPDIF on my Asus Essence PCI card was better than the one on my (8 year old) motherboard but that may have been expectation bias. It’s analogue output was superb.
Hi Mark,
You could simply ask someone in the trade to assemble something for you. Set a budget or minimum specification and you would have something suitable quite quickly. 3.5 inch disks seem unnecessary, the solution could be quite small if you went with 2.5 inch disks but apart from that a PC and a USB to SPDIF converter or sound card would be quite simple to do. As stated the biggest issues will be appearance and noise. I have a silent solution, no moving parts with internal storage measuring 240x240x40 that can sit with my kit or be hidden. You can of course go smaller or bigger. It is up to you. A ROCK compatible solution would be the simplest to look after. After that a Long Term Support version of a Linux server program like Ubuntu 16.04 would be the next best bet. It can be set it to do security updates automatically so logging in would be minimal.