

Yes, it’s possible
You need a SIP trunk to connect to and a PBX server. I would also recommend a proxy server to obfuscate your SIP server as it will be constantly attacked.
It doesn’t technically need its own network, but having it on its own VLAN is recommended as you will want to have some QoS policies for the UDP voice traffic otherwise your call audio will be choppy
It really depends on how much you value your time and how good you are with configuration
A QNAP or Synology will work and be pretty simple to configure out of the box. Installing custom software is possible, but can be tricky as they require you to enable sideloading and custom apps can be hard to find. Both have supported app stores with available apps to do what you’re looking for (QNAP has apps for both torrents and Plex. Not sure about Synology)
However, you will get way more bang for your buck by building one from scratch using something like TrueNAS and the Arr stack, but this can require a fair bit of technical knowledge about configuring containers and securing network services(Especially if you want them to be accessible remotely)
Most people here do selfhosting as a hobby and as a result, the time spent trying new configurations is negligible as it wouldn’t be much of a hobby otherwise.