The computer is the easy part — any 10-year-old Android phone would do the compute part, and the networking part, and the screen part; and would be easier to program; and would cost less than pretty much any SBC kit.

The hard part (at a DIY level) is, honestly, the physical buttons and knob — wired as an input device to the compute without hogging the device's (likely only) port — and with good industrial design (no gross 3D-printed textures that easily get dirty from being touched a lot; no deep wraparound plastic bezel that prevents you from touching the corners of the display; dynamic-function side buttons thin enough to allow the screen to show the labels for them; etc.)

Right now you can get an Amazon Kindle Fire 10 HD for $75 - it has a screen ~3x the size of the CarThing. Spotify works out of the box on it. Same with Web Apps. You can put it in Developer Always On mode in about 10 seconds via the menus and it works great as a dashboard.

The actual Car Thing runs Linux on an Amlogic S905D2 (quad Cortex A53) with 512MB RAM/4GB flash and an 800x480 screen. So you could do something similar with pretty much any random cheap ARM SBC.