The front element of most lenses is very strong and you can clean it with confidence. Just do not put solution directly on the lens since it is not sealed. I prefer to keep clean the lens and not the filter plus the lens.

IS will give you 2-3 stops so if your subjects are not moving it is a big advantage. I would go for this telefoto lens:

Canon 70-300 f/4-5.6 with Image Stabilizer, for a half an arm.

The Canon 17-80 I.S. is a good companion lens to that telephoto and has a perfect range for travel.

The L series will give you better quality but if you cannot afford them try to improve the picture quality by shooting RAW and post-processing with software like Lightroom, Aperture, Photoshop.

Shoot the same picture in aperture priority at f/8 and f/2.8, if the quality is similar you have a good lens. If f/2.8 looks soft, you now know why the L series is so expensive.