Performance wise, the older Genesis 1000-5000 are probably a little better due to the deeper fire box and two rows of flavorizer bars, however they are a bit more costly to maintain with the more costly flavorizer bars. Then again, they are typically a little cheaper to buy in the first place. I think that if you keep your eyes open, you can find a good candidate for $50 or less. If you are in a rush, a little more is good as long as the main parts of it are sound and the only parts you might need to replace or do significant work on are the burners, flavorizer bars and cooking grates.
Look for rust on the frame on the left side of the fire box where the screw attached the fire box to the frame. Also, look for warped areas on the fire box, especially along the front edge and right side above the burners.