When you go to buy a new door, make sure it has a composite sill, an adjustable threshold, and I would suggest steel over fiberglass because it is more efficient. Most steel doors come with a magnetic weatherstripping which makes it seal tighter. Also there is more insulation inside of a steel door since the steel skin is thinner than the fiberglass skin. I would also make sure it has a polyurethane instead of polystyrene. Since the polyurethane is more dense and fills the whole core your door will be more efficient and less prone to rust caused by condensation. Plus if you do it this year you can get a tax credit on most entry doors which is 30% of the cost of the door. The only upside to fiberglass is it dent resistant and won't rust.