Avent Bottle Warmer

I want to purchase a second hand Avent bottle warmer but I have no idea on which 1 should I get since they have quite a few models. Also, can I use the warmer to warm my breastmilk in packets? Or better pour in bottle? Can any mummies here please advice? I've already bought the natural avent milk bottle for my baby who will be due in early March. #advicepls