The parts coming from the floppy are likely to use 12V or 5V, minus the driver drop.
The way I guesstimate the voltage of unknown steppers is apply a voltage to 2 windings out of the 4 available and wait for the temperature rise. Then I slowly increase voltage until the temperature stabilizes around 70C (some steppers can take more but without a datasheet it's best to keep a safety margin).You can probably use this technique for the solenoid/dc motor, maybe with lower temperature rise.
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