Im a Canon guy, was going to refer you to this lens, I am sure theres a similar one for nikon maybe check the link anyway as there are sample images and maybe you can find a sigma or nikor with similar specs.
http://www.the-digital-picture.com/R...ns-Review.aspx
This famous photog works for nat geo etc. You can checkout his equipment here, all nikon:
http://www.joemcnally.com/blog/whats-in-the-bag/
If you find something of interest but not certain if its worth the money since most lenses are expensive, check out borrowlenses.com
I have used them, they are dependable and fast with shipping. You can rent that type of lens for like 150ish for a month to try it out or less for less time.
You can also check out usedcameras.com as they may have a more expensive lens used for cheap. Their stuff is in great condition and you can buy warranties on used gear with them. In this economy, any money saved is worth it amiright?!?!
Best of luck