It depends on what you’re doing with DSP, and to a lesser extent, the size of your library.
If the mini is your core then you need to have things like ssd storage i3/5 processor etc. if it’s just running as a bridge then any old macmini will be fine
I have a i5 8Gb with a hybrid SSD/HDD drive and it works fantastically - even HD audio.
Copy that, you don’t need much to run Roon. A solid state drive is nice It’s not really nesassary and a waste of money if your going to store music on it. I’m feeding my 2011 MacMini with music via a FireWire 800 external 3TB drive.