i can recommend the d700 brilliant and excellent value for money
FF Nikon has to be D700. Ergonomics great and specs still awesome. Yes it isn't a resolution monster, but 12 megapixels is enough for me. It's also going so cheap nowadays it would be a crime not to try FX this route.
For me the D600/D610 body and plasticky feel made it dead on arrival, shame as the downside of the D700 is its a bit heavy.
Not used the likes of the D4 etc but the build quality of the D600 is totally fine in my opinion , never used mine and thought "this is so plasticky" its a cracking camera