You've got the idea perfectly, rigid 29er with WTB dirt drop. Get disks, they're much better brakes, they don't wear out the rims, disk pads wear 2 or 3 times longer than rim pads, they're unaffected by water, and they're not expensive anymore. Get mechanical disks such as the Avid BB7, they're just as good as hydraulic without the hassles and vulnerability.
There are lots of racks nowadays that work with disk brakes. I like seatpost racks for the rear. They have many advantages, easy to mount, do better in a crash, the panniers sit higher, further away from the heels and ground and more behind your body out of the wind. And, of course, they're disk compatible.
A Karate Monkey is a great bike.
If you want something cheaper check out the Dawes Bullseye or Deadeye on Bikesdirect. They're sold as single speeds but they are "gear ready".
The Bullseye, $379, is disk only. The Deadeye, $339, has rim brakes but is "disk ready".
They are both use the same chromoly steel frame and fork.
After adding gears it would be half the cost of a Karate Monkey.