It sounds like you have three choices here in your price range to get yourself the better body: - A used D700 - A new or refurbished D600 - A new or refurbished D7000/D7100
The first two give you better low light performance, but they're also going to make you run out for more lenses pretty immediately after you purchase them so you have some FX glass. The third choice is less expensive, but in order to get the low light capability you'd have to run out and get faster lenses.
Personally, I'd go for #3, probably with a D7100, and go and spend the rest of my budget on faster lenses to give me the low light capability. I'd also buy lenses with an eye towards using them on FX later, and not get any more DX lenses, with the idea of in 1-2 years adding a D600 body as well (the D7100 and D600 have nearly identical control layouts and capabilities, so are a great pair together).
That's just IMHO though, hopefully food for thought.