Depends which circuits you are attending, generally the longer the better. In this price range there's a few 500mm sigmas available used in that bracket but they are in slower 6.3 range. Best bet for me would be a used sigma 100-300 f4, has the range for the majority of club circuits in the uk and is still useable at silverstone and donnington with a good constant aperture, and it's got great optics.
If you're looking at canon lenses then a 200mm f4 will suit a few circuits but may be a bit limiting due to small bikes and large run offs. Fast apertures should be a primary concern so look for f4s ideally, you probably won't get faster for less than 7-800 used