I would stick to a pre owned DSLR. I prefer Canon but Nikon makes excellent gear as well. If you go Canon, I would stay with the xxD series cameras as apposed to the smaller Rebel series, the xxD are more durable.
You can find the 20D, 30D and 40D between $250-$500 on CL, either of these would be a great way to get started. Put the money into quality glass, also good deals on CL.