Yes at that price you can surely give the motors a try, and probably would work.
Regarding the mosfets yes, they interrupt the gnd line so you can have a load with a different voltage. But it has to have the same ground, e.g. both 24v for stepper and 12v for heaters would come from same power supply so all the gnd returns to same psu. This is because the microcontroller gnd has to correspond to the heater gnd in order to operate the mosfet gate. Otherwise the mosfets would have to be optoisolated (which are not).
Regarding the mosfets yes, they interrupt the gnd line so you can have a load with a different voltage. But it has to have the same ground, e.g. both 24v for stepper and 12v for heaters would come from same power supply so all the gnd returns to same psu. This is because the microcontroller gnd has to correspond to the heater gnd in order to operate the mosfet gate. Otherwise the mosfets would have to be optoisolated (which are not).