Both the A2 and the N90s have been discontinued so you'd have to check ebay etc. I've had good personal experience buying used equipment from B&H Photo so you can check with them as well. That's one place that you really should call up and talk to someone because their salesmen know their shit. Their prices on the N90s run from $349 to $479 depending on condition, and the A2 is a bit cheaper. There's also a camera called the A2E which has eye-controlled focus. Basically there are 5 AF sensors which on the A2 are either manually or automatically selected; on the A2E you can manually select the sensor by looking at it. Kind of a gimmick but it does indeed work.

As for lenses, if you're going to be shooting trackside then you don't want one of the cheaper consumer telephoto zooms. Their maximum apertures are crap for action shooting especially when you get to the long end of their focal length range. I would go with a used fixed-length telephoto lens made by an aftermarket company like Tamron, Sigma or Tokina. That way you can get better optics and a larger max aperture for your money.

*edit* But um yeah, it'd still probably be a bit more pricey than you want. In the Canon area (since I haven't checked out Nikon lenses for ya yet), Sigma makes a 300 mm f/4.0 lens that is almost 700 bones new but you might be able to do a bit better used. Check the B&H used dept and see what they got.