They were trying to send this link and I’m going to strongly disagree with them - that system is a substantial downgrade from a Steam Deck. The GPU is a GT 1030; on top of being Nvidia, it’s 8 years and 4 generations old and was bottom-tier when it was new.
That said, the idea is sound. Buying an actual gaming desktop PC from a few generations ago can be a very budget-friendly option, but shipping an assembled PC is a nightmare for multiple reasons, and even more risky secondhand. If you’re going to buy a used prebuilt PC, find one locally and pick it up yourself, don’t have it shipped to you.
Download all the music you like, whether buying it on Bandcamp or Amazon or pirating it for literally any reason. If it’s too much media to keep locally on your phone then host it on a computer in your house with one of any number of hosting solutions (Jellyfin, Plex, Subsonic, etc.) and VPN your phone’s internet connection to your home network so you can stream them from anywhere without having to set up and secure another method of remotely streaming it. If you were on Android I’d recommend Symfonium, but I don’t know anything about the iOS app ecosystem.