70-200mm f/4l has great reviews. It captures less light than your 18-55 when you're at 18, and more light than when you're at 55.
70-200mm f/2.8l captures more light, but is about $950 used, so out of your price range.
70-300mm f/4-5.6 IS is also a pretty popular lens, with extra reach and a fairly small difference in image quality compared to the 70-200 (which is sharper than the 70-300). This lens captures about the same light as your 18-55.
I am not a Sigma enthusiast, but most people who get their 70-200 f/2.8 EX (around $700) seem happy with it. The reason to consider them over Canon is because they offer a faster aperture (f/2.8) for less money.
Another Sigma model (slightly out of your price range) is the 100-300mm f/4.
These are the lenses in your price range that best fit what you seem to be looking for. If you can find the 55-250mm f/4-5.6 IS lens that's not available in the USA, you might consider that one as well. Since you don't want primes (no zoom), I wouldn't suggest the 200mm f/2.8 (about $500 used) or 135mm f/2 (about $700 used)
For a general idea about lens capabilities, there are several websites to look at, including:
1.
http://www.photozone.de/8Reviews/index.html -
2.
http://www.popphoto.com - search for specific lens reviews -
3.
http://the-digital-picture.com/Canon-Lenses/ -