The most essential lens to have is a 300mm F2.8. It's standard for runway shooters, and it also makes a great lens for outdoor shots. The background is nicely blurred, and the shots have a very unique look to them. A bit expensive, so you'll have to wait on that one.
The other essentials for fashion and portraits are the medium telephoto primes, 85mm 1.2, 105mm F2, 135mm F2. The quality of these three lenses can't be beat. Again, some expensive glass, and best with a full frame camera.
For now, the single best lens to get would be a 50mm 1.8. A friend of mine shoots Canon, and she said the 50mm 1.4 is cheaply made, so not worth spending the extra money.
Another good lens for now would be the 18-135, like you mentioned, or get an aftermarket 18-200, Tamron makes a good one. Not sure if Canon makes an 18-200.
When you get into it more seriously, look at the 85mm 1.2 as a good step into high quality glass.