The biggest difference in image ‘quality’ is down to the sensor size rather than anything else given that, as supernovae says, all dslr’s are pretty amazing these days.
The 7000 is a great camera but it’s still has a dx sensor, the d600 is a large fx sensor version of the 7000. I know you’re probably locked into the dx lens system but you could pick up a good second hand d600 and 50mm lens for £400-£500. ( then trade in old camera and lenses for other fx lenses?). The larger sensor gives you a larger field of view, so a 50mm lens will get as much in the picture as a 35mm on a dx camera before distorting and image crop options give you a couple of ‘effective’ telephoto lengths. Bigger sensors aren’t about better detail they’re about seeing things ‘more biggly’
After shooting on smaller dx sensor cameras the step up to fx is like moving to medium format, the difference is night and day. The pictures will look more professional more because of this than any other technical characteristics save the lens quality (and the 50mm fx lens is just about the best lens there is and it’s cheap as chips owing to its ubiquity.)
As Matt suggests, MBP are a great place to get a camera from, properly checked over and excellent reviews. I think the best option is to go for one of their ‘excellent’ rated second hand camera with a very low shot count.
