I own a Rebel XTI and have been a Canon loyalist for a while but lately have gotten into some professional work, mostly shooting outdoors and sometimes in lower light. I am doing a lot of active portraiture and need about 200mm of telephoto but use my wider range at about 50mm too. My question is; What is one of the most versatile and best quality lenses out there that would work well in various situations. I've been looking at some of the Canon L-series and the ones that seem most versatile are too expensive for me, but if I am gonna spend a lot on a lens, which lens is best for the least amount of money, or where is he best place to buy lenses with the best deals? If I could spend 500 dollars on a lens, which one would be the ultimate? Where should I get it? What about the Image Stabilization technology? Are there any places that sell used or refurbished lenses at good prices? Just some purchasing advice, please! Thank you so much.