What premium Halloween item do you recommend?

If you have Martin, I suggest buying his wizard skin for 50 donuts. It's the least expensive way to add more help in fighting monsters and earning event currency. For 75, you could buy the Beastarium which earns 100 wands every 4 hours, I think. Alternatively, you could wait for Suzanne the Witch & her Gingerbread House (full character and building) which will be 100 donuts after rebate (140 regular price). It's available on October 7th.