Frankly, looks more like its a heart problem more than a head problem
Change your heart if its a heart problem
If its a head problem then lets look at it this way:
1. You are comparing D600 (entry+) with D7k (good mid-level)
2. There are 2 more equivalent options 60D and 7D. 7D is known to have fast AF speed.
3. You are comparing kit lenses on both cameras or different? This can affect AF speed.
4. Is AF speed so critical? 1/2s, 1s, 2s difference? To make all this change worth while?
On the plus side, your logic is 'sound'. If you can sell off at $1700, You loose a bit, but you get rid of a heart problem which can eat into you.
On the other hand, you only have 1 lens rather than 3 lenses.
Think over it, if its worth the cost and trouble.
Try out the 60D and 7D first, before deciding.