I am a newbie but have decided to purchase the D300. I want a camera that will last for years. I am struggling with understanding lenses. Would it be better to buy just the D300 body for $1799.95? They offer two other packages. One is with a 18-135mm lens for $2099.95 and the other is with a 18-200mm lens for $2479.95. Can I get a better lens if I purchase it separately? Thank you!