First about the D750, I have owned it for over 3 years and it is a FANTASTIC camera!!! In my experience its bullet proof reliable, wonderful low light performance, excellent AF system, tilting screen comes in handy for low or high shots, its a gem

Now regarding weddings, I am an event photographer and my D750 was my main camera for many weddings, its just really is a great fit for weddings, mainly because of the 3 features I mentioned before, the low light performance, auto focus system and tilting screen.
From my experience I would not recommend the D8xx for weddings over the D750.
I also own the D850, which is just amazing too in its own way, a big step up from the D750 but surprisingly the D750 is still my main body and the D850 is second body, mainly low light performance, I am not too impressed about the D8xx low light performance and low light performance is a BIG deal for event photography!!!
You could down-sample the D8xx but that's an added step and when processing 1000+ pictures for a wedding its just adding hours more of unneeded work.
As for what to add to the camera, 24-70mm 2.8 and preferably also 70-200mm 2.8 plus one prime maybe 50mm 1.8 for extreme low light situation.
I want to agree with "Peeb" remark about shooting weddings for beginners.
Beginners has no place in shooting a wedding, this is not a joke, you need to know what you are doing, a person that shoots a wedding needs to know how to use his camera, then he needs to know composition.
After you learned the basic (this can take years of practice) you need to find a mentor that will take you under his wing, a wedding has its work flow, its rhythm, only after you shot few weddings and got the feedback from your mentor and you proved to yourself you can do a whole wedding alone only then you can take the responsibility of shooting a wedding.
Also remember doing photography today is great as a second career but not as main job, market is saturated with kids who own T3 and D3200 and feel the fact they own a camera makes them ready to shoot weddings, they put an ad on kijiji and ask for nearly nothing just to be able to start working, competition is crazy and while its possible to get a job it is very hard to live off that, just my 2c