What are my chances to revive a (gasoline) pressure washer after ~ten years?
You let it run dry, which was key. This means you don't have smelly, gooey, inflammable fuel dried in and clogging the small carburetor passages. 

Check the oil level and fire it up. If it's lubricated, it's probably just fine. If there's moisture in the oil it'll evaporate when the engine comes up to normal operating temperature. 

There's no reason to change the plug unless you have reason to believe that it's fouled, but storage doesn't do that.