Perhaps you have to make a judgement here. The fact that the D3000 means you can only use AFS lenses has resulted in a pretty expensive lens shopping list. If you move to a body that can take advantage of screwdrive AFD lenses then not only are they cheaper than the AFS variant but are far more accessible on the used market. Another consideration re the body is that the prism viewfinder of the likes of the D300 is far superior to the Pentamirror type in the D3000, so even the manual focus you mention is a bit of a different world.
On the used lens front I have bought from Grays of Westminster before, and no doubt there are other dealers where you can save serious cash by buying ecx+, exc++, to mint- over the cost of buying new. I have bought lenses from Grays in mint- condition, and the only way I would know they were not new was the lack of the origional box. Example from Grays listing today, Mint- 50mm f1.4AFD for £185 compared with the best price I have seen on a quick look for the new AFS version of that lens £288:99.
AFD is not quite as fast as AFS but then it is fine for studio work. I have the 85 f1.8AFD and a quick check on Stevie's D300 and a/f is easily fast enough for portraiture of a moving subject.
I don't know how fast your subjects are moving, but remember the likes of Bailey, Duffy, Donovan, Avedon, Litchfield and Parkinson, to name just a few, did not have access to the fancy wizz bang kit available today. I would hope they would inspire you. Look at their images and lighting plays a key role. Far more than the lens that was used.
As for focal length then it does depend on working distance, and how much of the subject or its surroundings you want to include. As you have an 18-55 you should be able to see if anywhere in that focal range fits the bill. Before spending on a 35mm set your lens to that and give it a go. You will soon know if that focal length fits the bill or not. Remember if you want shallow depth of field for subject isolation, then within the bounds of your available working distances, the longer the focal length the better. Wide aperture of course helps too, and in this case f3.5 on the 85macro is not particularly wide.
Don
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