Hi
Go for the 89, provided that condition is ok. Its like buying a car, get the latest model possible.
They are fantastic bike's, thats why every one here rides one. Ive got a brand new one(2006) and love it. They are very forgiving bikes but haul ass if you want them to. I get to ride +-4 times a week in awesome surroundings. Yesterday was the first day that i really gave it stick(still too new, dont want to crash it just yet), and my God, was i surprised. They are really quick if you pay attention to revs and ride hard. You will not be dissapointed.
My kdx is not the most powerfull that ive ridden, I do have experiance on large bikes, and im impressed. I feel so much more comfortable on the kdx than i do on my mates yz250wr, therefore i get better times on the kdx.
thats just my experiance.
Cheers
Dylan