If you're going to own one macro lens, I always recommend that it be in the 100-mm range. Your best purchase is always going to be the professional-level lens made for your camera (Nikon, Canon, etc.). However, those lenses can be very expensive. In the case of Canon, it's their L lens. They also used to make, and it's still available, a non-L macro lens, which is superb. It was their only 100-mm macro lens for years. It sounds like Nikon has a similar setup, though I don't pay attention to that brand. If you don't buy the camera-brand lens, there are two third-party lenses that I think are worth considering: the Sigma EX 105 and the Tamron 90. Both are extremely good values and you don't have to search far to see that they both can produce excellent images. The only thing to note is that, when you focus with these lenses, the "snoot" extends. For some that's an issue. I once owned the Sigma and never found it to be a factor. Most of my work is now done with a Sigma EX180.
Autofocus with macro photography is a non-factor. You need to manually focus your images because, more often than not, autofocus will be fooled, particularly with reef photography. However, some type of autofocus is desirable since you can use macro lenses for "regular" photography, too. Remember that autofocus is a relatively new capability in the history of photography. For many years, photographers captured excellent images with manual focus. It's not that hard to do, particularly for macro work. Editorial comment: The word lazy comes to mind.
Depth of field has nothing to do with sharpness, as a general statement. With macro photography, you're more often looking for a lens with the widest range of f stops so you have as much depth of field capability as possible. With closeup/macro work, you're frequently looking for more depth of field, not less. Understanding and controlling depth of field is an entirely different subject that would take days to discuss. Where sharpness and aperture come together is when you discuss lens quality. Professional lenses will be much sharper at both ends of the aperture range than "regular" lenses. You pay for the ability to shoot a lens wide open and have the image be sharp. It's pretty rare that you want to photograph with the aperture closed down because funny things start to happen with light in the lens barrel when you do that.
The most important piece of equipment, when it comes to getting sharp closeup/macro photos, is a tripod. You can't do this type of work well unless you use a tripod. This is proven time and again with just about all of the photos that are posed in the forums. Very few are in focus/sharp.
Hope I've been of help,
Gary
