To play Wii's Skyward Sword on Wii U, is a Wii Motion Plus and Nunchuk all what I need?
In particular, do I also need to buy a Wired Sensor Bar?
You will also need the Wired Sensor Bar. When you are launching the game, there is a step where you have to calibrate the Wiimote with the Sensor Bar. After that calibration you don't need it anymore (except if you want to calibrate your Wiimote another time during the game phase), but it will be needed each time you launch the game again.