have you researched the AF system in the D610? I'm not saying its not any good, but I've got a feeling its the same as what's in the D7100 (but could be wrong), and you said the AF was one of the things you weren't happy with...
Edit - just found this quote on the interweb - "Since Nikon reused the 39-point AF system from the D7000 (MultiCAM 4800) on the D610, my first task was to see if there is any difference in AF performance between the two. After testing the AF system of the D610 alongside the D7000, I came to the conclusion that the MultiCAM 4800FX AF system is more accurate than the MultiCAM 4800DX AF system on the D7000, especially in low-light situations. While AF speed seems to be about the same on both, the D610 does not hunt for focus as much as the D7000 does and the hit/miss ratio is much better."