I would recommend a fast telephoto lens..
24-70mm 2.8 (if you're gonna be somewhat close to the runway.
Or...70-200mm 2.8 if you're obviously not going to be too close to the runway.
Fashion shows usually have low light in the room itself and the stage is blasting with light to illuminate the models so everyone can see them clearly.
You're best option is to go with a fast lens so that you will be able to open the aperture wide enough to get a blurry background, use a low iso and a fast shutter speedf to get all the action without blurring the models movements when they catwalk down the strip.
I hope this helps.
Nikon D300S + MB-D10 Battery Grip
Nikon AF-S 16-85mm VR 3.5-5.6
Nikon AF-S 35mm 1.8
Sigma APO 70-200 2.8
Nikon Speedlight SB-600
MC-36 Shutter Release