^ what he said.
Do research before. A camera store can give you info, but it's likely that they don't have a clue about what you are trying to film, and are just looking to hock some gear on you.
If you think your kit lens has plastic elements in it, you need to learn up on camera gear in general before you start thinking about purchasing! get familiar with canon's line up of lenses, what makes some lenses better than others (aberration, sharpness, contrast, color rendition, build, mechanics of focus and zoom, etc). Then read a ton of reviews and view a ton of sample images and make comparisons.
THEN, decide if you want to fuq with older manual focus lenses and used lenses. You're shooting video, and at that resolution even softer lenses look sharp. If you're ok on that, start looking for lenses you can buy canon mount adapters for, used, on craigslist or ebay or KEH.com (when you order, call and talk to Dan Orchard, the only legit way to use that site). Some of those lenses look great on video because the softer focus and (sometimes) higher contrasts of older, simpler lenses look more natural. Plus, they are more prone to flare, which can be a good thing or a bad thing (I like it.)
you may find that your next purchase should be a fluid head, or maybe something else that's not a lens at all.