I have a chance to buy a used Canon 100-400 L lens for about $850. I've never purchased a used lens so I'm a bit hesitant I looked at it and it looks good. Didn't see any dust or fungus in it and it looks well taken care of. The date code on it shows its an April 2005 version. Do you guys think it's a good idea to buy such an old lens? Any tips or feedback would be appreciated. Thanks in advance.