You may like the cost of the D610 a lot better.

If size and weight aren't a problem .. like MM says, wait for a good lowly priced D800(or better yet D800E) to hit the market.

D610 feels almost identical to the D7K both in terms of body size/feel and weight. I know the D800 is a lot larger, and feels a lot heftier .. more accurately solid!

I dismissed the D600(back then) based simply on the smaller overall molded size of the grip compared to the D800.
Had got used to the D300 sized grip after a couple of years with a D70s.
For personal reasons, I found that the smaller grip of the D70, and hence D7K and D600 were much less comfy when holding the camera for extended periods.
D300 and D800 are 99.9% perfectly contoured to fit my hand!

While MM is right in that you have a large investment in current lenses .. it looks like most of not all will go with the D7k with a move to Fx.
They will all still work on a D600 of course, so you won't have a desperate need for new lenses, but I suspect that you will want some more anyhow.

Sigma 10-20 and 17-70 will of course only work in crop mode .. but saying that, you'll still get a bit more image with the 10-20mm if you still shoot in Fx mode.
You can crop a 1:1 square format if you need want too, with still good IQ.
Have no experience with the 17-70 tho .. so can't comment on it's possibilities.

If you were to follow this route(of getting a D610) and you did get a single lens to go with it .. I'd recommend a Nikon 16-35/4 VR.
It will fill the wide angle void left by not having 15mm equivalent(or close to it) across the entire frame .. gets you to 35mm, which is not far from the 50mm prime you already have, and of course you have the 70-300mm lens still(that still works on Fx) .. not the ideal lens for this sort of camera, but it should be usable to a degree.

Tammy 90 should work well too.

Unless you really 'need' the 36 Mp of the A7r, and hence the D810 or D800 .. the 24Mp of the D610 will be fun to play with. Of course in some situations, more is merrier .. but don't automagically think that once you have more, you instantly have more to play with!
Your lenses have to be good to be able to use those pixels.

Quick tale of my great little Tamron 28-75mm f/2.8. I used to use it on my D300 without any real thought to it's downsides. it produced nice enough looking images that I was happy with, and it seemed many others seemed to like a bit too.
But mounting it to the D800E .. !!!! .. it became a bit of a beast .. ugly, covered in all manner of aberrations. Vignetted madly into the corners to the point that even at f/11 doing a panorama stitch wasn't an option! The vignetting in the corners affected the exposure between one frame to the next frame . ie. uneven colouring in a plain blue sky as one result.
Note that this was at f/11. I've never come across any lens that caused vignetting at f/11 .. in fact by f/5.6 I don't think any lens does!
Sad tale of a favourite lens of mine was that it had to be retired. I still have it, won't sell it and it can take a good shot but not in all circumstances.