As a newbie to Roon, Raspberry Pi, I’m a bit confused as to what Pi-related hardware to order, e.g.
I plan to use a usb-connected dac.
If I want to use ethernet would the PoE hat be advisable? If so I assume I don’t need power supply hardware.
A suggested case for above…
What DAC are you using? That’s a good start. 
None yet. I figured I’d choose one from the list of those tested. Unfortunately, I’ve been told that Roon Essentials doesn’t support Roon Bridge which is apparently needed to connect to ropieee. So my interest is somewhat diminished - 
I guess it all depends on what you want to do and if you outgrow Roon Essentials capabilities or want features most of us enjoy.