The target should be one that produces a good match to the final printed result. Your perception is influenced by the ambient light, that's why you have these options.
6500K is what most people use. Some find 5000 better, or anything in between.
140 cd/m2 is almost certainly too bright unless you're outside in the sunlight. The recommended starting point for a "normally lit" environment, whatever that is, is around 110-120. Personally I find that too high and have set mine to 90, but then I like to work in rather dim light.
Some monitors don't perform well at low luminance settings. I had a Dell U2410 that could only reach as low as 120 at minimum brightness (but that had to be calibrated through the video card, no direct communication with the monitor). Now I'm using a pair of Eizo Flexscans that go down to 80 comfortably, and I'm sure the NECs will as well.