my Canon 24-70 L series works well at fashion weeks - both here in Melbourne and in Sydney. It can be beneficial to get to the show early to get a good possy. In the 30 years I've been doing them I've been stuck within touching distance near the entrance of the runway or 25 rows back! I've even done one during a powerstike - the fun of the late 70s - with candles replacing the lights!!! (Pushed B&W film to 6400iso)
P.S. don't forget there are second hand lens's out there. People are starting to turn their equipment over like computers and cars after three years. Pro gear should last a while and if you go to somewhere like Camera Exchange or Michaels in Melbourne you will get great 2nd hand gear with a warranty.