I assume you mean less than $500, and if you only want stills, look at a 40d, 50d, or 1d mark ii

(I use the "n" version for stills). Your 18-55 will work on the 40d or 50d, not on the 1d, but Either way I would suggest you get a new lens. You can get an older tamron zoom, its f2.8, 28mm on the wide end (so not super wide on a crop), and its either 70, 75, or 80mm on the long end. They all exist, but one is much better, and I think sub $300 used. I don't think it has super fast AF like canon USM/"L" lenses, but it should work for most of your stuff. I'm not sure if the better AF of the 1d will help, or if it would be a waste and the lens is the same on all 3, maybe someone else will know. After that, 70-200 or 11-16/fisheye if you want.
If you have trouble deciding between the three, here is a little breakdown.
40D - 10MP, 6.5 FPS, Cheapest
50D - 15MP, 6 FPS, still cheap but newer version of 40D, does video with magic lantern
1D Mark II - 8MP, APS-H (instead of APS-C like the other two), 8.5 FPS, built like a tank, too many AF points to count, the n version has the biggest screen, and I think it is the most expensive of the three