I just bought a second hand computer from my local “recyclery”. They sell used computers from the municipality in my town here in Sweden at very reasonable cost.
I gave €200 for the Dell Optiplex 3050 SFF and got a Samsung 970 EVO Plus m.2 NVMe drive and a couple of 8Gb Cruzer DDR4 RAM sticks (not necessary, there was already one 8Gb RAM stick installed)
I installed the m.2 disk and RAM and then it took the whole of three minutes to boot from an existing Roon USB install drive, and then install ROCK! 
When Roon was running I shut the PC down and installed my WD Red 6Tb media drive in place of the Dells original Hynix 128Gb SSD. Unfortunately I needed to initialize the 6Tb drive from ROCK’s GUI, even though it was formatted with ETX4FS previously. Something to do with keeping the ROCK experience seamless and appliance-like I was told.
However, once initialized it was an effortless task of connecting to the ROCK server from a Windows 10 machine holding a copy of my media library. A couple of minutes spent on a robocopy script (and turning Roon Server off from ROCKs GUI) and copying was on its way. It took 15 hours to copy almost 5Tb of media over 1Gbps Ethernet.
This morning I started Roon Server and then restored a backup of my database.
A small adjustment of the Storage paths had to be made but identification and analysis went smoothly.
At lunchtime today I had a running ROCK server with an Intel Core i5 6500, 16Gb RAM, 250Gb PCIe m.2 boot drive, 6Tb media drive and a built in optical drive for ripping! It works great and punches really well as a host for 150K tracks and around 11K albums in various resolutions.
Performance figures shows upsampling of RedBook to 24/192 at 50x processing speed, to DSD128 at around 8x and to DSD512 at 2.2x processing speed.
It is nearly quiet, except when ripping, and draws around 15W when idling and about 25W when upsampling to DSD128.
In short, an excellent Roon Server and ripper at very low cost!
Oh, I added its data to the Performance Matrix linked in this post: