Nikon fixes all D600. Even outside warranty.
Just avoid gray imports and get the original bill for the camera, then you'll have no issues whatsoever.
Besides, I love that the D600 doesnt enable the monitor when you change the ISO. The D610 does that, as does the D750. Thats the one thing that drives me crazy about my D750.
Btw, the other thing about my D750 is that lifeview is much more useful thanks to the 100% magnification on a single button press button, but unfortunately choosing the AF point is hella slow.
Ge the D610! You'll love the low light performance, its a fantastic camera. I've been using it for over a year now and still amazes me how good the low light performance is. Sure, it doesn't focus as good as the D750, but it has yet to be an issue for me.
The D750 has a couple more advantages than just better AF, though. Like the wonderful flip screen, the wider AF field, the magnification to 100% in one single button click.
The D750 is the camera the D600 should have been in the first place.