If you are looking at basic internet usage: youtube, netflix, browsing the internet, streaming music and remote desktoping then chromebooks are suprisingly comptetent for the money.
Make sure you get a model that supports Android apps for the added functionality.
Most chromebooks have very limited local storage but many also support SD cards allowing you some expansion. You can set the SD card as your download location for your mp3's but its worth noting that not many android apps will currently see the SD card if thats how you were planning on playing back the music.
A chrome app called "enjoy music" works well with the SD card though
here is a
link
I have an Acer R11 which may also suit you. Its fairly cheap, has a reasonable keyboard and I often see over 10 hours between charges. The sound quality via the headphone jack is great, though if you were planning using Bluetooth speakers you may wanna head over to Reddit ChromeOS and see if someone else has the same speakers as Bluetooth music streaming seems a bit hit or miss.
Cheap chromebooks are a great thing, I also own a Pixelbook which basically performs the same functions as my R11 just for a lot more money. I'd say if you are looking for cheap capable devices chromebooks are the way to go. Once you start spending more you get very little more for the money and you would be better rewaded with a windows device