I have made exhibition prints as large as 80x100 cm (I usually crop to 5x4 proportions) from my D700, only not at the highest ISO values.
More pixels would be nice but as long as you don't have to crop in post, 12 MP works surprisingly well as long as they are clean pixels. With careful noise reduction in post, it is possible to make 50x75 cm prints at ISO 6400.
The only difficulty in upgrading to the older body is the metering. The D700 was the last Nikon that didn't protect the highlights, so it is easy to blow parts of the photo if you compensate your exposure in the same way as in a more modern body. Matrix metering is less developed than in later bodies too, and misidentifies situations more often. This can make the transition a bit more difficult, but not too hard...
Have you factored in that most of your lenses are DX only and that you will need to upgrade those too with a move to FX?
The D700 is a very good compact pro body that is still not replaced in Nikon's lineup and probably never will be...