The D700 is a marvellous camera. I've always though the D3/D300/D700 generation of Nikons had the nicest straight-out-of-camera colours. I could never get colours that pleased me quite as much from my D800e (though that was a magnificent camera in its own right).
If the D700 is still producing the pictures you want, I'd say hold on to your money.
At only 30,000 shutters, it's still got a long life ahead of it.
I just sold off my two D300s and they had 210,000 and 140,000 shutters respectively (and we're still going strong ten years after I bought them). Nikon build their kit to last.