The D810 can be seen as two cameras in one, a FX 36mpix and a DX 15mpix combo. It really can replace and upgrade you D300s, with much better ISO but a bit less FPS (5fps on FX and 5:4 modes, 6fps on 1.2x and DX modes, without the grip; and 7fps in DX mode with the grip). I believe the D810 is really worth it, and you will be able to use it for many years to come (200k actuation cicle), just like you did with the D300s.
For lens, I almost got the 14-24mm, but in the end decided to go with the Nikkor AF-S 18-35mm f/3.5-4.5G IF-ED. Optically, it is second only to the 14-24mm, very sharp corner to corner while wide open, and way way way lighter than the 14-24mm, making the 18-35mm ideal for travel. The 18-35 also can take 77mm filters, while the 14-24mm needs a way more complex solution to take filters. Going to 35mm adds great flexibility as well. The 18-35 costs 60% less than the 14-24mm too.
Here are my sample shots with the D810 and the Nikkor AF-S 18-35mm f/3.5-4.5G IF-ED version:
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Nikon 18-35mm f/3.5-4.5G ED AF-S - an album on Flickr
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Flickr: The *18-35mm G & D810, D800, D800E - Nikon Nikkor* Pool