Hi Afira, Here are some suggestions and resources for that area.
1) As you may know, many river bottom lands flood periodically around there. So beware buying flood plain land unless you're specifically looking for a challenge. Take a look at the NRCS soil survey when looking at properties.
http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
The county geodata information will also prove useful for designated wetlands (although they are not always accurate).
2) When buying land, consider the growth plan for different cities in the area. I grew up in unincorporated King County (Juanita), near Bothell and Woodinville in the 80s. That area has completely changed since I was a kid with subdevelopments everywhere. Where there were rural horse farms are now big box stores and apartment buildings. Juanita got eaten by the city of Kirkland and is now "north kirkland". I believe the city of Duvall has a limited growth plan, so you may want to look around there.
3) Are you familiar with the growing season there? If not, check out the Maritime Northwest Garden Guide and Growing West of the Cascades.
4) Consider taking a permaculture design course with local "old growth" permaculture folks like the Bullock brothers.
http://www.wildernesscollege.com/permaculture-summer-semester.html
5) Are you new to the area? Learn about wild and native plants by checking out books like Discovering Wild Plants (Scofield), Plants of the Pacific Northwest Coast (Pojar/MacKinnon), and Medicinal Plants of the Pacific Northwest Coast (Moore).
6) Learn about the watershed of your area. Is it fed by snowfall? Is that threatened in the next 50-100 years? The summers are super dry, so consider the long term water situation of any land you may buy. Ponds are difficult with the silty soils (I think the bullocks use pond liners) west of the Cascades. So consider that water is a huge design issue with any land in Cascadia.
7) Learn about the Native People of your area. Did you know the territorial governor of Washington, Issac Stevens, was going to give small reservations to native tribes and then (in his final solution) round them up on to the Tulalip reservation? Where did the natives on Whidbey Island go? Why do Native folks today get half the salmon, half the wild hunt, and 33 of the shellfish? You can read about all of this in such books at
People of Cascadia (Bohan),
The Bitter Waters of Medicine Creek(Kluger) ,
Land Use, Environment, and Change (White),
Keeping It Living (Duer and Turner), and
Message From Franks Landing . All of these books are likely available at the amazing King County Library system.