Simon,
Thats a pretty wide range of subjects so from the Nikon range I would suggest either the 105mm f/2.8G AF-S VR Micro-Nikkor or as nirofo has suggested the 70-180mm f/4.5-5.6D AF IF-ED Micro-Nikkor for a bit more flexability. Naturally there are three lenses ( 105mm, 150mm and 180mm ) from the Sigma range some or all of which forum members could give you a view on. I am concentrating on A/F versions so as not to add to your difficulties by touching on manual focus.
Nikon lenses are more expensive than their Sigma counterparts but you can save a bob or two by buying secondhand Nikon from a reputable dealer like Grays of Westminster who will give a 12 month warranty.
Link to Grays listing
http://www.graysofwestminster.co.uk/...secondhand.php
Without a doubt the most veratile is the 70-180 Micro Nikkor. This is no longer a current item so you are in the used catagory for this one. Grays currently have two £925 & £975.
If I remember correctly Andy153 has one and should be able to give his views on it.
The Nikkor 105 AF-S VR is current but yet again is available used from Grays £445 & £525. As this is current there are quite a number of reviews on the net. KR even has a side by side size comparison shot with the AF-D version.
No longer current is the 105mm f/2.8D AF Micro-Nikkor. Once again available used at Grays. My friend Foxy has this and I can confirm that it is a good performer.
Sigma have versions in 105mm, 150mm and 180mm and there is probably a larger number of users on this forum of Sigma Maro lenses than Nikon Micro lenses.
One thing to consider is that those lenses without a built in motor such as Nikons AF-D versions can accept almost any extension tube or bellows unit to give greater magnification. AF-S versions require a special tube containing electronic contacts to allow control of the aperture. This may be an important consideration if in the future you intend taking shots of very small bugs.
I have not put a pic up for a bit, so here is a hoverfly from earlier in the year, taken with 105mm lens at f11 so you can see how shallow the depth of field can get even on a subject of this size.
Don