Used lenses have dust! Its the least of things to be concerned about, and, if you get fixiated on looking at dust, you might miss the important things like autofocus or decentering.
Its pretty easy to clean most lenses, but tougher to fix a autofocus motor or IS. Realistically, its not worth considering a used lens unless you save enough for a average repair and still have a 10% margin. I won't buy one unless its at least 30% off retail, usually I go for more.