For the really cold nights, I would go with a heavy blanket with a separate fleece liner. I don't like to layer more then two blankets. More then that and they can get bunched and cause rubs and sores on your horse. Your best bet would be to ask people who have lived in the area for a while and also have clipped horses.
It is just so hard to tell. You have taken away any ability that your horse has to keep himself warm, so you really have to keep a close eye on them until you are sure that you have the right combination. Some horses run hotter then others and are uncomfortable and sweat in heavy blankets, other are very sensitive to the cold and just can't get warm enough, no matter what you put on them. Either way, it needs to be right or you can be creating other health issues.
You don't have to be crazy to work in a barn, they'll train you!