The D700, D600 and D7100 are all wildly different cameras.
The D700 is the pro camera of old, a smaller D3, which was the first full frame camera of Nikon, with the same sensor. That means the D700 has the best controls and build quality (the D3 would still be an upgrade even over that, though). However, back then DSLRs didnt have video - I think the Nikon D90 was the first DSLR with video ?!?
The D7100 is an APS-C camera. It has a top notch Autofocus, just like the D700, but otherwise its very much a D600 in build and interface. As an APS-C camera, the sensor is small (24x16mm vs 36x24mm for full frame), which means all focal lengths get a crop factor - a 200mm lens on an APS-C sensor gets the same field of view as a 300mm lens on a full frame sensor. This makes this camera predestined for Wildlife, where you can never have enough range.
The D600 is an entry level full frame camera. It has the same resolution as the D7100, but with a more than twice as large sensor area.
Glas for FX is a LOT more expensive than for DX. But theres also a LOT more choices, and you'll get quite a bit more image quality for you extra money, too.