The Nano PI is a 10$ small board computer, 300€ is the total cost of the setup with the 16 ports switch, 2 x 5 USB ports power adapter and ETH/microUSB cables, advantages of using a Nano PI is that you could just add more 10$ boards and increase the possibilities, octacore boards may be an even better option.
This is the DIY Kit of the OKAY 2 Synth. It comes in two varieties: BYOP (Bring Your Own Printer): Just the PCBs and electronics components. You'll need to print the 3D models yourself. Complete: Everything included, no 3D-printing required. Just solder the PCBs and assemble! Recommended for folks with medium to advanced soldering ability.