Okay, so I only plan to buy only one lens (for now). What would you recommend I get? I would like to limit myself to $700-$800, but because I know that lenses are interchangeable with cameras I will spend more if I can find something I know will last me. I would like something with auto focus, & would like 200 mm in length, but I'm open to suggestions.. The faster the better. I'm looking for the best quality I can get. & I also plan to buy a 50mm lens in April, so let me know what you think! Thanks!