before you go and buy that 50/1.4, you might want to set your 18-55 lens to 50mm and see if you are comfortable at shooting at that focal length. if you are comfortable then by all means get that lens.
but for me personally. i would make do with the 18-55. it is a motor show. the cars aint gg anywhere. so 18-55 gives you the flexibility of it zoom range so you can shoot at different focal length. and dont forget your external flash
Depend on what you are shooting, the motorcars or the babes.
If you have the space to move around to compose, prime lens will give you the bokeh
and sharpness that you desire.
However, for motorshows usually they are crowded with photographers especially when you are shooting a popular and beautiful model. The zoom lens will give you more chances of getting your shot. The 18-55 and 55-200 VR are good lens to start with.
Try out a 50 f1.8 first, and see if that suits your needs.