what kind of fashion photography u going to do??
catwalks? portraits? fashion products?? studio or outdoor??

dun just go for one lens. go for the one u can used the best out of it. try more different lens combinations. u might be able to find something u really like. that going to be your style. fashion photography has no limit.

for me, im a Canon guy, my fav. is 85mm, 100mm.

good luck.