Chris, a couple of things to consider when choosing a lens for a C size CCD, which Nikon uses, first is that the lens you use will have an image magnification comparable to a 35mm of about 1.5 times. In other words a 200mm on you d50/70 etc is like having a 300mm on a 35mm camera. I not saying that the image is comparable in all areas of image quality etc, its just that that the image has the same angle of view when comparing an image taken with a 35mm + 300mm and d50 + 200mm. As the camera body uses a smaller image sensor, you will have an increased depth of field in comparison to the 35mm lens, f stop vs f stop. This is good because you can open the lens more and still achieve a good depth of field, in low light conditions etc.
Secondly, you are photographing fast moving objects, so you need to make sure that the camera and lens work as one and make sure that when the lens not only stops down it actually can focus on the object quickly enough. otherwise you'll still have only half a car in your frame and it will be blurred. Some of the budget lenses you refer to have a very slow focusing response time.
You may also like to look at the respective specs to compare focusing modes, ideally choose a camera body that has S C & M modes. These stand for single, continuous and manual. The continuous is very handy when panning moving objects, as you can compose, set expose and focus all in one step by half depressing the shutter button, continue panning and when you have the view you are after, take the shot, the image will be crystal sharp, because the camera body is adjusting the focus as you pan. Another reason to choose a lens that has a good focus response time.
Another consideration is to think of buying a good small telephoto zoom and then use a teleconverter like a 1.4x when you need the reach of a longer lens. VR is very worthwhile getting, unless you are well practised at hand holding shots. Have a look at any of the professionals and most if not all will use at least a mono pod for those shots.
One last thought, using a digital SLR body, get some time practising with it before LM, the metering etc needs getting use too and is not like using a 35mm SLR, where possible use manual for all your shots and get some control.