I think your reasoning for getting a D600 (assuming the rumored specs are similar) for an IR conversion is sound.
Of course, your path would be one of a double early adopter. You will have the D600 before almost anyone else and presumable be one of the first to IR convert it (which would essentially void the warranty).
You mention the D700/D3 has a a known (but minor) issue for IR conversion. What will the D600 have?


Brian