"Electromagnet 5kg DC 3V 5V 6V 12V" - what do these multiple voltages mean?
The total power requirement (watts) will be similar for the different voltages. If you pick a 6V one it will draw about 4x as much current as a 24V one. 

The higher the rated voltage, the more turns of wire are used, and the wire will be thinner (higher AWG number). Since the magnetic force is proportional to ampere-turns the number of turns needs to be 4x larger at 24V compared to 6V.