Anyone have any suggestions on a good starter lens for night shots with my existing flash?
Your photos will have a more natural look to them if shot with a large aperture instead of the flash. So my advice to you is: get a cheap and fast prime lens.
The 50mm 1.8G is quite cheap, so is the 35mm 1.8G. The 85mm 1.8G costs a bit more, but it´s really good and might be the thing for you, if you cannot get close enough to your subject. The D series 85mm 1.8 is really good too, if you do not mind the rotating focus ring during AF. You might get the D series 85mm quite cheap now, when the newer G series one is out.
I really don´t know what kind of images and framing you are after, so I cannot give you more exact advice on the focal length. I have never been to a drag race, so I have no clue how close to the action it is possible to get there. If you have a kit lens, as you probably do, check the EXIF data and see what focal length has produced the kind of framing you are looking for. Get a prime close to that focal length and you´re done.
My 0.2 cents.