I came her looking for the same thing, though I may be able to answer your questions based on what I am doing rn. I have an older Mac Mini that I use as a "iTunes" music server. It is embedded in a console stereo system and I access it in two ways: 1) I use the "iTunes Remote" App on my iPhone / iPad, and 2) I use Screen Sharing on my MBP to remote into the Mini to manage the computer, fix song names, etc. You can configure the computer to auto-load ITunes when it boots and not look for, or load, other apps that aren't needed. You will need to configure Home Sharing in "iTunes", now "Music" on newer Macs. I have multiple Airplay devices around the house connected to various home theatre / amps, etc. It is a very powerful music server with great distribution controls (much better than Sonos (in my opinion, yes, I've worked with that quite a bit and the sound is not as good as what I am accustomed to). If at all possible you should have your Mac Mini and any of the Airplay devices connected via Ethernet cable. They all have WiFi connectivity but most home network WiFi systems won't handle all of the throughput if you have a number of Airplay devices and other computers, IOT devices, etc. running at the same time. 
