Hi, I wish to buy a used Sigma 120 - 400 mm lens, as the cost will enable me to upgrade my Canon 300 mm f2.8 L IS lens to the mark 2 version. I also own a Canon 70 - 200 f4 L IS, Canon 100 mm f2.8 L IS macro, Canon 18 - 55 mm IS USM, two Canon 70D c...