I have a Maths and it’s an essential module if you are building a Eurorack system. But it definitely requires a Eurorack power supply and case, which will eat into your budget.
I would also say an Arturia Keystep or Korg SQ-1 if you are looking for some kind of MIDI to CV utility.
Finally, the $29 Lightning to USB Camera Adapter is awesome for connecting your iPhone and OP-Z over USB.
Or, fight the capitalist system and don’t buy anything!