My sister had a house that was heated with electric radiant panels in the ceiling. they loved it, and the cost wasn't horrible. The panels were white, but may not suit your style. Note, these made the person feel warm, and while they did heat the air and structure some, not as much, so placement is critical. WIth a room over an unheated garage, the floor could end up being really cold. I think to make this comfortable, may take some additional thought. radiant floor heating, depending on planned covering might be an option. you didn't mention where this is located, the weather extremes, or the type of windows. Single pane, and your heat load will be huge. You need to do an analysis on what amount of heat is needed, then decide the most efficient way to install it. Is there going to be water available, would a small boiler be an alternative? Could you get gas there?