I have a Mac Pro 2010 tower for sale + 23" Cinema Display.
2.8ghz quad core, 16GB 1TB HDD.
I think it is running Sierra at the moment but can put High Sierra on it if you want.
No keyboard or mouse (although I might be able to scare something up if it is a dealbreaker).
£375 cash on collection from OX29 vastly preferred as I don't have a box for the monitor.
I do have a (slightly tatty) box for the tower.
For an additional £100 I can include a 512GB PCIE OWC Accelsior card that makes this a much faster computer.
I was still doing albums with this until a year ago when I moved to a 5K iMac.