I've only owned the 2002 KDX220R so I can't speak from personal experience, but... My brother-in-law races in a 4 hour endurance race (two person teams) every year at a local ski hill. He rides a new WR426F, while his riding buddy has a beat up 1990 KDX200. His buddy logged faster lap times this year (like in previous years if I remember correctly). In short, the rider is what makes the most difference. You should be fine with anything after 1990. I'm not knocking anything older than that, I just have no insight to what the KDX was like prior to 1990.
As long as you find the bike fun to ride, then it will be the right choice...
Have fun, and ride safe..
-Mark-