I recently did the same thing, moving up from a DR350SE to a 2000 WR400 kicker, with the 3.2 gallon tank.
For folks that need to get your bike plated, look at the TUSK DS kit, it was just less than $200.
The Tusk kit is similar to the Baja Designs kit- very straightforward to install with no big surprises other than how easily those cheap mirrors break at the mounting bracket during installation.
I actually superglued the mirror bracket, and have a different set of cheap plastic mirrors on the way from Amazon.
The Tusk kit doesn't come with a headlight, but it does include a tail light. I didn't use it since the WR400 already had both.
The WR400 also has dual filament bulbs and the extra wiring for the front and rear, so it's already set up for you to add the brake switches to the existing hardware.
In Nevada, I needed front and rear brake light, so I ordered an additional hydraulic switch with my order (avoiding separate shipping charges).
I expect the bike will stay mostly stock except for a few things: an exhaust upgrade to FMF or similar system,
I'm also going to add a AC/DC converter so I can run the DS kit and a GPS or cell phone off the bike power.
A new Moose Racing rear fender pack is already on the way, and I picked up a Panoram cycle computer (similar to the Vapor), but haven't mounted it yet,
That will help with trip mileage (fuel) and speed.