They are £150-£250 on Amazon right now. Cheaper if you buy elsewhere and wait 2 weeks for delivery. $400 is not a realistic price, are you looking at something else?

If you are going down this path, an N150 machine is cheaper, more flexible (Windows), more readily available, brand new, and performant enough for all the above use-cases. An old Mac Mini makes no sense to me.

Are modified ebikes popular where you live yet? They can be very fast, very quiet and will do a lot of damage if you collide with one.

Check out https://gitlab.freedesktop.org/gdesmott/karapulse for a full Spotify Karaoke experience, this does use lyrics pulled from Spotify (via GStreamer's spotifylyricssrc and librespot).