I have owned a D610 and currently own a D7200 and a D810. I shoot lots of birds, wildlife, landscapes and seascapes. Although the D7200 is great for action and portraits at low ISOs, the noise, even at ISO 200, is troublesome. I'm getting rid of the D7200 and will stick solely to my D810, there really is no comparison. As for the D610, it is a great camera, but it too does not really compare to the D810. $2K for a factory refurbished D810 is a good deal and I have found refurbs to be as good as new the few times I've purchased a refurb. One thing about the D810 you need to know is, it is much more like a medium format or even a large format camera, especially when you're shooting at ISO 64. You often need to use a tripod. Keep in mind also, that when you get to f/8 or higher you begin to lose the benefit of the lack of OLP filter. As for lenses, like everyone else I recommend primes, but I primarily shoot with two zooms most of the time and the results are extraordinary: the Nikon 70-200 f/4 is the one I use most, especially for landscapes. What's really good about it, aside from it being as sharp as most primes in its zoom range, is it focusses and zooms internally; like a prime it does not suck dust and moisture into your camera and onto your camera's sensor. The second lens I highly recommend for birds and action is, and some people are going to rail on this, is the new Sigma 100-400mm. It is an extraordinary lens that I have compared to my Canon EF 100-400L and it is as good or better, and the price is way more affordable. It is smaller and lighter than the big zooms like the new Nikon 200-500mm (I returned mine, it was just too big and too heavy to carry around when shooting birds or wildlife—great if are staying put and keep it on a tripod though). And if you want a big zoom, I recommend either the Sigma 150-600 (I've shot with it extensively on my D750 and on my Canon 6D), or the new Tamron 150-600 G2, that I own for Nikon. I do think the Sigma is a better value though for the price.

If you can afford $2K stop reading this and go buy the D810!