I have been into DSLR photography for about 6 years now. I'm a Nikon guy but everything I say can most likely be applied to a Canon product.

The body is not important. You mentioned a used D40. That was my first DSLR. It's a fantastic camera and to this day some of my favorite pictures have been taken with it. I sold it a few years ago.

The lens is where you make it or break it. Super zooms in the sub-offensively-expensive price range are slow lenses, meaning they don't have a very large aperture to let light in thus requiring a longer shutter speed. Longer shutter speeds mean blur. Nikon made VR to help counteract the "shake" of your hand(which causes blur). It physically stabilizes the optics in the lens with a gyro. It helps tremendously. I would not have a slow lens without VR.

Believe me when I say you don't want to carry around two lenses. It's a colossal pain. If you just have a 70-300mm lens it will be great for zoom but nearly unusable for shots where you're trying to catch a landscape or a small group of people. And forget about using it for pictures in the cockpit. You would have to carry two lenses.

The alternative is a great all around lens, 18-200VR. It can take great wide angle shots and still has enough zoom for distant subjects. Like all the other "cheap" lenses it's slow and not great for low light situations without a tripod.

Any lens with an aperture of 2.8 or less(large aperture is a smaller number) is going to have an advantage in lower light situations. Also, a large a aperture gives you more control over DOF(depth of field). This is the effect of making your subject pop out on a blurred background.

One more thing to keep in mind if taking pictures from the cockpit, camera lenses are made with the highest quality optics but when you shoot a picture through the windshields of an airplane your image quality will drop dramatically due to all the imperfections and heating elements in the windshields.

Hope this helps a little