There have not been any rumors about a D800 upgrade/replacement. Several years after launch it is still by far the highest resolution 35mm format camera available and packed with features from the top pro cameras for 1/2 the price. Coming from a D80 you will be blown away by the extra resolution and light sensitivity.
Regarding lenses, the 28-300 fared surprisingly well in DXOMark tests. The downside is that it is larger and heavier than its DX equivalent 18-200. I used the 18-200 on my D200/D300 for many years. It was as good as or even better than any of the DX kit lenses and offered great versatility. If you are used to that lens and happy with its image quality then the 28-300 will do the same job on FX but it will not extract the full potential of the D800 sensor.
I now use the 24-120 f4 on my D800 and it offers a very visible improvement in image quality over my 18-200. It is about the same size, a little heavier but faster, build quality is better and it is visibly much sharper throughout the shared range. I also use the extra 4mm at the wide end far more than the long end so I don't miss the extra reach of the 18-200, I have other better lenses for that anyway.
D800 files are huge in comparison to D80, especially if you shoot RAW. So unless your computer equipment is reasonably up to date you will struggle with slow image processing.