Which HID coversion kit should I get?

You will have to do a projector retrofit to be considered HID legal. You will have to open your headlights and place these OEM HID projectors into the Sentra. Since you have one bulb doing both low beam and high beam. You should get the FX-R combo package.(second link). This projector has a shield to do the high beam as well. It will be more expensive but you will have Legal HID and you will no blind other drivers with the glare of HID Conversion kits.