I wouldn't buy the D700 if it was to shoot with this lens... if you intend to buy additional lenses it could be good.. If I was you I'd wait for the next nikon announcement, the D700 prices will drop.
Also, there's nothing wrong with buying used as long as you know what you're buying... Most of times that mean using it before buying.. so ebay might not be the best choice but you could watch for local ads via craigslist or kijiji.
If you buy on ebay, make sure you get some video of the camera in working condition.. and watch the seller's stats too.. I wouldn't send that amount of money to a seller with under 500 feedbacks.. and with good rating (good rating, depending on how many sales, usually means over 99.6%) You could read ebay guides about this.
Oh and, Welcome to the forums... if no body welcomed you yet.