As I have had some time to further the project, I've done a redesign of the board (see the pictures section on the left) with room for two Teensy microcontrollers, and a header for a Pimoroni Pirate Audio (or other Raspberry Pi compatible I2S audio hats with optional screens) or a Raspberry Pi and whatever hats you want to use (forgoing the need for the second Teensy).  Additionally there is a 20x30 prototyping area, and mounting holes that match up with 3.5" bottom screw positions for mounting (or you can use right-angle hardware.)

The previous OLED menu system is being brought out to a separate microcontroller to allow for some more advanced features.

Once the design has been validated, I'll upload the new design to the project files.

For $50 you should be able to build your own basic board with just the NCR5380, backend Teensy 3.5, and minimal passives / connectors.

For around $100 you can fully populate the board with two Teensy microcontrollers, and a front panel LCD / Audio module.

HDD sound simulator:

The frontend Teensy can generate simulated harddrive sounds (and eventually CD & tape mechanical sounds too!) in realtime, for those who miss such things.  There is a sample recording in the Files section.

CDDA emulation:

You can have the frontend Teensy play back wav files from it's own SD card, though I might implement a pass-thru for the backend to send audio that is embedded in a bin/cue image.  With a Teensy 3.5, this uses 16-24% of the CPU time on the frontend controller.  The frontend Teensy can be a 3.5, 3.6 or 4.1.  The 4.1 should be powerful enough to decode MP3 files in realtime, and can do S/PDIF audio (passthru to your sound card!)  The backend however still needs to be a Teensy 3.5 to work safely with the NCR 5380.

240x240 UI:

Using the Pirate Audio hat, the screen can be much larger than the previous 64x32 or 128x32 OLED displays, and in 16-bit color too.  There are multiple versions of the Pirate Audio hat, one with a line-out, and another with a small speaker.  The UI on the front will let you mount/unmount images, change IDs and modes, control CDDA playback, etc.  Some basic UI elements are implemented already to allow changing the volume of the CDDA and HDD sound simulation.  I'll be uploading a video of the updated UI soon.