What should I be aware of when buying a zoom lens (other than that it will fit my camera)? What specs should I pay close attention to?
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1. The zoom factor of course
2. Aperture, if aperture value is low you will either be able to take better photo in darkness and/or do blur effect in lower focus distance
3. Min. focus distance, you will not be able to take a correct shoot if subject is closer than this distance
4. Weight depending of your need you might not want something too heavy

Each time you want to buy a lens, try to find some review on Internet !
If you know someone with the lens you are interested in, or a similar one, borrow and use it if you can. That goes way beyond reviews!