Hi, I've got a 2001 and 2004 KDX 220. Everyones right, if you buy a new bike take it apart and grease everything. I don't care what brand. This is one item all the manufacturers skimp on.
Then get the stiffer fork springs. The rear shock spring should be fine unless you're a real big boy. Unless you're a "B" or "A" level rider don't bother with revalving. Just turn the clickers and make sure you have the sag set right.
Next, get the right set of bars that fit YOU, so you don't get blisters on your palms or rub the skin off your thumb or knuckle. Get a set of Bark Busters to keep your fingers in the shape they started in. Install a grease zerk to grease the fork stem bearings and the rear brake lever pivot. Take a correct size bolt and drill a hole the length of it and then install a grease zerk in the head end. You're going to use this to grease the Kick Start Lever pivot by taking out the set screw and screwing this in. The Kick Start Lever and Rear Brake pivot points will wear out in a season unless you do this and grease them regularly.
When ready add your favorite pipe and muffler. This depends on your type of riding and skill level. If you want to go for a carb mod upgrade, it does work but avoid the head modifications unless you're going to do some serious racing. If you do a head mod with higher compression, don't take off the air directional vanes from the radiators and put on guards, because the bike will over heat really bad. The cooling system can't handle the extra heat without the air directional vanes.
If you find the lower shock mount bearing wearing out really often, contact me and I'll tell you how to fix that. It's a poor design by Kawasaki that they've carried over for more than 10 years.