If your telescope only has a 1.25" focuser, you cannot really use your D750 for AP. I have a D750 and it requires a 2" nosepiece. 1.25" is too small for the full frame sensor of the D750, and you get vignetting.
Here are some pictures taking with a D750 through a 1.25" nosepiece. See how the image is really just a small circle in the frame itself? That's because the 1.25" is too small.
The last adapter you linked to is what you need: https://www.amazon.com/gp/product/B00T4K9EM2/ref=oh_aui_detailpage_o08_s01?ie=UTF8&psc=1
If your focuser cannot accept a 2" barrel, you will have to get a 1.25" version, but will end up with vignetted images like I posted above.