I concur with Danny, above, there is little to choose between the different makes in optical quality; Sigma/Tamron/Tokina will give you results that are as good as a Nikon lens in this category. The difference is in the build quality.  Having said that the gap seems to be closing all the time. I use a Sigma 150mm which I consider a superb lens both optically and the build. 

You mentioned you need plenty of DOF and this could be a deciding factor in any decision. DOF alters with how close the subject is to the lens, as well as focal length and aperture.   Therefore macro is almost by definition shallow DOF.  To maximise the DOF you need to pick the shortest focal length that is suitable for your needs. With flowers in an indoor setting there is no problem increasing the shutter length and using a tripod so you can use a small aperture to maximise DOF. There would be a problem if you want to snap critters though, they will move and a tripod is at best awkward, so there comes the compromise - a focal length long enough to shoot before you get too close and scare them away,  you will need a high shutter speed to match the increased focal length and freeze the action or camera shake. 

Focusing I have never seen as a problem - most people use manual focus with a macro lens, because of DOF issues, set the focus to roughly the right place and then move the camera slowly back and forth to get the final adjustment is the method I use.  

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html