Headphone amp and DAC quality: for the price, it’s brilliant. In more absolute terms, I’d call it ‘pretty decent’. 
The Pi does have a 100Mbit Ethernet port.
When it comes to the OS, you have a few choices. You can use IQAudio’s image (which is RoonReady and ready-to-run) or use DietPI, allowing you to install RoonBridge and a specific audio kernel for PCM up to 384.