There are three features to look for with a label rewind unit.
Motor power. As labels come out of the printer and are rolled onto cores, they can become very heavy. Smaller underpowered rewinders would struggle to turn the motor.
Core size. Labels are typically rolled onto three sizes of core. Make sure you get a rewinder with the correct sized core, or buy one with an adjustable one.
Tension Adjuster. The Tension Adjuster means the label rewinder smoothly takes up the printed labels – without undue stress on the labels, printer or rewinder motor.