which he would sell....
Julian are you looking more for a zoom lens versus a prime or doesn't that matter? Will you be filming night-time stuff? The sigma 30mm 1.4 or the canon 28mm 1.8 might be good options to look into. the 30mm on ur camera is almost the same as a 50 on my camera (full frame) and it is by far my favorite lens size and is almost always on my camera. If you're ok buying used you shouldn't have a problem finding either in your budget.
The Tamron 17-50 is a good option as well, but I cant speak from experience, just from others reviews. Pretty sure MikeBlatt or Abe Krieger is selling one right now
If you had a bit more money I would say tokina 11-16, but maybe that will be a future purchase for ya.