This is a challenging ride for any size bike but worth it for the views over the Taneum area from the numerous balds along the route. I would say intermediate rider and up. A steering damper helps on a ride like this.
The challenge comes from the zillions of rocks you are going to ride over on the Northern part of the route. That said, if you are a veteran of Section 2 of the WABDR, this section will pose no problems whatsoever.
This 166 mile route starts in Cle Elum. Check out some of the other routes, particularly the Central Cascades Sampler, if you want to see some more interesting ways of getting to and from the route.