If it is large home and there are walls and furniture, you probably will not be able to cover it with a single access point. External antennas do not increase the output power: they change the "shape" of the radiated power; so if you have a directional antenna, it will put more power in one direction and less in the opposite direction. You are better off using multiple access points or use external antennas if you want the coverage to be "shaped" in a way that does not resemble the 105s coverage.