Check in your Wii U's box, there should be a sensor bar in there. Also, if you plan to use the game pad screen to play Wii games on, you don't need a sensor bar as there's on built into the gamepad.
Motion plus isn't that important, but if you're buying controllers now you might as well get one with it built in. It's probably not any more expensive and sometimes you get better performance than with the add on (not to mention the extra bulk the add on will cause.)
The nunchuck is very important, you definitely should have one.
Make sure you get real Nintendo stuff, sometimes the third party stuff is really bad quality and either flakes out or just doesn't work with certain things. I'd recommend staying away from Chinese sellers, they're cheap, but also often fake, even if they look like 1st party stuff. To be safe, I'd say stick to major known local or online retailers. It'll cost more but you know it'll work, and if you do get a bad one (it happens) you can return it.