The 18-55 lens is certainly not the best lens, and any lens that slow will suck in low light. We cannot give completely accurate advice without seeing some of your work, however I would guess that your experience is now limiting you more than your lens. Better lenses will make your photos better, but only when you have reached the limitations of your current lens.
Lighting in fashion photography is more important than lenses. Whether its a full studio, a strobist setup, or just some reflectors and sunlight, lighting cannot be overlooked. Check out Strobist 101, this is a good way to get started in lighting without spending too much money.
When you do want to invest in more glass you have many choices.
Depending on your working distance, many people will be vehemently opposed to using lenses 50mm and wider, I do not agree with this personally, after all its all about you style. IMO 50mm is a good choice for wider shots, sometimes even 30mm and wider.
For head shots anywhere between 75 and 150mm is common, both the 85 and the 100 would be a good choice, the 100 is not as fast as the 85, but it is a macro lens, so its quite versatile. Although primes are king in fashion, don't discount the use of high quality fast zooms, especially if you want to get into other types of photography, like spots or photojournalism. A 70-200 2.8, or f/4, is an extremely sharp zoom, its great for all types of photography including fashion.
Hopefully this helped a bit...or just made thing more complicated.
We are happy to answer any more specific questions that you have.
