One point you may wish to consider is the lens choice available to you if you are considering a Nikon D40.  You have mentioned third party lenses and I don't think these will auto focus on that particular model of camera.

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