Wow, Halloween is over two months away. You plan for this far in advance, don't you?
Any razor that can shave your face can shave your head. As you are not ready to commit to going smooth, and are doing this only to play a character for one evening, I don't think there is any point in investing in any new devices for shaving. The skin wig actually would be better for what you want. They really are not expensive and are not difficult to put on properly.
If you are determined to go smooth for that night, however, I suggest allowing a barber to do it as you will run less risk of cutting yourself and ruining the appearance of the character.