I'd start by finding out whether that Sigma is good enough for your standards. Or at least good enough until you can save up for the 24-70 f/2.8. (And, yes, you really do want one of those!)
Note that what luminous-landscape is recommending is that you use "the highest quality lenses" with the D700. Well, you have ruled that out (no second job!) It's not clear that the 24-120 is not among the best of the alternatives.
If not the 24-120, how about the Nikkor 24-85 f/2.8-4? $565 at B&H. AF rather than AF-S, but from reports I've seen it's a decent performer.
If you're interested in used, I keep hearing good things about the AF 28-105 f/3.5-4.5, and it's insanely cheap on the used market.