It's better to get a external flash first. Especially fashion runway models will be moving about. With a flash you can freeze the model movement without setting high ISO. Thus your image will not be grainy.
I believe you can shoot without flash on most fashion runways. Their spotlights are designed to maintain a constant white on the models and are usually bright enough... Correct me if I'm wrong. Of course, I'm not referring to those ad hoc events in shopping centres.
it depends what kind of fashion runway you are referring to?
- the real proper fashion runway?
- or the normal kinds that are openly available in shopping centers?
if the latter, the lighting might not be ideal, and a flash might do you more good, but then again 500D+18-55, you might not have enough range.
Ask yourself which you need more
Fast lens - stop motion, do not affect ambient lighting, does not distract the subjects, thinner dof due to large aperture used
Flash - stop motion, affects ambient lighting (but can overpower ugly color cast caused by lousy location lighting), distracts subject, you can stop down your lens for more dof
Depending on the type of fashion events you are shooting. Basically if you need a flash, you need it anyway and most likely you will shoot in FP flash if you want speed for fast moving models. If you need a bigger aperture for the depth of field, flash or IS is not going to help you on that. Try to find out what you will be shooting as well as the lighting condition. Most people bring both along like a f2.8 with a high gn flash.