It's overly simplistic, but the D800 essentially took the pixels and processing of the D7000 and expanded coverage to an FX sensor. The D7000 is probably more demanding on technique than almost any camera previously released by Nikon. It also has had lots of complaints about softness - largely technique related - so there is some merit in the issues around higher resolution. Based on that experience, the D800 will probably have some softness complaints but they will be mitigated by the hands of skilled users who have been "warned".
The D700 is a fine camera, but even beyond the sensor there are a number of enhancements in the D800. The AF system or video capability could by themselves be reason enough to upgrade. And the D700 remains a good camera.
If you are printing below 10x15 inches, the D700 has plenty of resolution. Even up to 20x30, the D700 image can be upsized with little loss of quality. There is nothing wrong with a used D700 or even a new one at $2199 or lower.
Depending on mileage, I'm not sure you need any kind of service or overhaul. There's not a lot to do other than clean the sensor and blow out any dust.