I think a Mac Mini is your best choice. I have been streaming my lossless ripped CDs from an iMac or MacBook Pro to an Apple TV or an Airport Express in 4 different rooms for over 8 years. Recently, I decided to set up a stand alone music server using a Mac Mini. I purchased a late 2014 Mini with 8 GB Ram and a 500 GB HD on eBay for $320. Setup was a breeze and it is performing flawlessly.
I doubt a Node2i will sound better. We tried a Node2 a while back and with its internal DAC, it did not sound as good as an iMac, MacBook Pro, MacBook Air, or Mac Mini streaming to an Apple TV or Airport Express with our DACs. We could not tell a difference when we tried the Node2 using our external DACs. It certainly did not sound better. So the internal DAC may be the weak point in the Node2.
I doubt a Node2i will sound better. We tried a Node2 a while back and with its internal DAC, it did not sound as good as an iMac, MacBook Pro, MacBook Air, or Mac Mini streaming to an Apple TV or Airport Express with our DACs. We could not tell a difference when we tried the Node2 using our external DACs. It certainly did not sound better. So the internal DAC may be the weak point in the Node2.