If it is the basic 300W DC spindle (they often do have a label, but it'll be where the motor gets clamped), then IIRC the control box ran around 24VDC. It's likely the original controller ran using the older TB6560 driver chips, which were only good for up to 40VDC, so these boxes generally ran a good bit below that to try and improve reliability.

I run the motors on mine from a ~45VDC supply, but they do get a bit toasty once they've been running for a while (the current is actually the key factor, as they'll only see high voltage when running at high speed). I'd say aim for 24-36VDC, as that will give reasonable performance for these machines. You'll likely soon realise the shortfalls of the machine, and a lower than ideal voltage is not likely to be one of them!