I do something similar - cds via ALAC to itunes.
If you get a Sonos connect you can connect it to your 2-channel Hifi.
You don't necessarily need a NAS, you could use your 5TB HD, but then you'd have to leave the mac on all the time. the NAS means you can shut down your mac and listen on the Sonos.
Then you can add active Sonos speakers later as you like.