A
as a starter, please suggest which is a better choice and why > D3100 or D5000
1. ND filter cuts down the light by a few stops (depending on the type of ND filter). So it is not at all recommended for use as "protecting the glass". It is used in scenarios where you need slow shutter speed where otherwise such a slow speed would have led to overexposed shot.
Yup exactly. If you need more zoom get the 70-300 VR I mentioned. Or if your need arises you can go for the Sigma Wide Angles (although a bit costly) Wide Angle Zoom Lenses - SigmaPhoto.com