I'm going to school for video, so keep that in mind. Sadly, I can only spend about $500 tops on a lens, since I spent the rest on camera gear. I wish I could get a 70-200mmIf you're going to school for video, and you plan on using the camera to shoot your stuff, then I'd spend your money on a few primes rather than on a tele zoom. You'll want something to cover the wider range, and probably something in the more traditional portrait range.
I guess a 17-55 zoom would cover your wide needs; something like the Tamron non-VC, or that Sigma lens. As for the portrait side, something like Canon's 85mm f/1.8, or the Samyang 85mm f/1.4 would cover you. The Samyang lenses are a great value for video use, since you'll be manually focusing anyways. Another option is to use some older lenses (Nikon AI, older Contax/Zeiss, etc) with adapters.
The 18-135 (either the older one or the new STM) would cover general needs, but, you'll find it limiting for video use if you want shallow depth of field. I would avoid impulse buying, you already have one good lens that is useful for getting to know the camera (which is the most important part); you can find out once classes are going what you'll need more of in terms of focal length