I love my D750 but I did not like the 24-120 f4. I thought I was tired of carrying around 2.8's (I had a D7000 and a 17-55 f2.8) so I thought the 24-120 would be a nice downsize compromise. But I found the focus tracking slow and the lens was much larger than I expected. The range to 120mm was surprising and a little strange since I was used to the the 17-55 (~24-70 in FF). In the end I sold the lens to KEH for pretty good money, about what I paid for it when considering the kit price. I bought myself a used 24-70 f2.8 and I love it. If you are only doing portraits than maybe the 24-120 is ok for you, I don't know if you'll miss the f2.8-f4 range or not. I love the D750, it is a huge step up from the D7000 in terms of high ISO, focus speed and overall awesomeness. For shooting my kids, you cannot beat the D750 in low light, great colors and AF, for the money that is.
As for buying used, I would not hesitate buying used from BH, KEH or Adorama.