Thanks for your replies, they have been most helpful! A Canon 20D certainly looks like a good one for the budget, as does the Tamron 90mm. Fieldwork will be taking place in the field, but to capture images of winkles - so not dead, but certainly not going anywhere fast
The thing I was least sure about was the stand - I have had a search for specimen photo stands but I haven't found anything
The camera will need to be positioned so the lens is pointing directly at floor, so suspended above the specimen. I would assume there is a stand that would fit the bill, i'm just finding it difficult to track down!
Once again, thanks for your help, it is most appreciated!