Sell your current system (Old processors have higher power consumption) , get an amd processor (Keeps the power usage low) based NUC (Go to ASUS workstation webpage and search , dont use a domestic user motherboard) or assemble one yourself. Customizing the OS or putting softwares wont have that much impact. Just dont install anything apart from Roon , antivirus etc. Keep the install minimalistic.
Regards,
Guru
So I did get my Ubuntu install working pretty well. Used it about a week. I took out the Ubuntu drive and swapped in another SSD ( I happed to have two of the same 500GB SSD) And did a Roon Rock install. So now I have a Mock. It was quick, I did it while my friends watched Avengers Endgame 
And everything is working just fine. I have to recopy my music to the 2nd drive on the machine now, and I have restored the backup I made from Ubuntu. So now I have two installs, Ubuntu and Rock.
I’ve added my video SSD as a USB drive to the Rock and I can use it for serving video to my Living Room too
I’ll see how it goes the next week.