Reddit mentions of Solomark 2inch Precision Ultrawide 48mm Camera Adapter for Nikon DSLR Camera and 2inch Telescope Focuser

That all looks pretty good, I think those are the right rings for your scope. For the camera adapter, since you have a full frame camera you might want something like this. It should fit right into the back of your focuser and has a wider opening than the usual 1.25" t-ring setup.

It doesn't look like that focuser has any place to mount the guide or finder scopes so you'll need to either run a second set of guide scope rings on top of the main scope or find a Williams Optics finder scope base of some sort that mounts to your focuser.

When you do the initial alignment of your scope every session you need some way to precisely find reference stars, so you'll need a diagonal and an illuminated eyepiece with crosshairs. It's also nice to have so you can visually observe things instead of just taking photos.

Sometimes there isn't a purchasable elegant solution to setting up your astrophotography rig so you end up improvising. If you don't have any space left to mount a finder scope as well as a guide scope you can get something like a telrad which just tapes to the side of your telescope. This was my setup a little while ago. The scope is the SkyWatcher equivalent of your telescope. The focuser has a dovetail built into it so I mounted the guide scope there then I mounted a telrad to the side for the purpose of helping with initial alignment. Later I replaced the telrad with a green laser.

Also, just so you know. If you decide to use an autoguider like the one you linked you'll need a computer running the guiding software. There are some stand-alone auto guiders like the celestron nexguide but I don't really have any experience with those so I can't comment on their usefulness. If you really don't want the bother of hooking a laptop up to your rig you can keep your exposures relatively short with a full frame sensor and good polar alignment - you might not even need to run an autoguider.

> Orion ED80 telescope and a Celestron Advanced VX Equatorial Mount

Okay, these two can be made for astrophotography, so that's good.

> but for DSO's I am uncertain about in regards to eyepieces with T-Ring capabilities

You absolutely do not need eyepieces for photography in this configuration. You need a 2" to T2 adapter and a camera-specific T-ring (actually, this one you linked would work too). Ideally you'll want a flattener/reducer for your telescope later to get better results, and some sort of autoguiding solution so you can make longer exposures.

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.