I am thinking about installing a Goodman 90 plus efficient furnace for my house but putting the furnace in my garage. The garage is not heated and gets pretty cold. If I build a closest and insulate it will this be OK because of the coils that have water in them, or will the freeze an ruin the furnace?

If you are going to put it in your garage I would recommend insulating the garage and having a supply to it, possibly even zoning it out. Otherwise I would recommend not to.