Questions about hardware setup

Hello!

I am the owner of a SqueezeBox Radio, I use it to listen to internet radio, I use mysqueezebox.com to edit my radio-favorites. This works fine, I am very happy with the device, it's very portable inside the house.

A few years ago I bought a 2nd hand Controller and two 2nd hand Receivers for a very low price, but after testing them (no problems!) I never used them. :eek:
Now, I want to listen to my favorite internet radio stations, AND my mp3-collection in different rooms, using the receivers.

I am going to need a server program to serve the mp3's. I am thinking about installing picoreplayer on my rpi 3b+

This is the hardware in my home:
(1x SB Radio, 2x SB Receiver, 1x SB Contoller)
iPhone, iPad, MacBook, Windows desktop computer(s), Synology NAS 110j (old one!), Raspberry Pi 3B+

1. Is it better to store the mp3-files on the NAS, or is it better to store them on an external harddisk, connected to the rpi? What are pros and cons?
2. I also want to play the mp3- and radio music on the rpi. Can I use the SB Contoller to do this, or do I need an app on one of my mobile devices? Maybe the SB Controller can be replaced completely?
3. The rpi is connected to my Edifier R1280T speakers using the rpi's audio out miniplug. Music played with the pi sounds ok, I am not an audiophile, but would you recommend a DAC? (Can I buy it in Europe?)
(both SQ Receivers will be connected to an amplifier).

Thanks.