I will resist the urge to get in a debate about saving trees (a renewable resource that employs many hardworking people in the area and actually stores carbon from the atmosphere rather than adding to it). But steel is an option, you would have a harder time finding a reasonably priced builder...
Yep.
I used standard 2x6 for mine, but there are additional structural elements planned (balloon framed 2nd story, lean-tos etc). One wall was 18' on 2' of extra stem wall. Also, I built in Stevens county, a lot different to deal with than Spokane.
If you are in Spokane County you will not be able to build a 20' 2x6 wall unless you use engineered lumber. 2x8 would probably be the minimum. You might get 16' plus a 4' stem, but 4' of stem wall adds a lot to the cost. You will have to have it engineered for sure. I think anything over 12'...
The weight of the walls or the things you hang from the walls aren't the problem. How many 3 story 2x4 buildings are out there? It's the lateral loading on the walls from wind (or earthquakes) that limit the height. Without some other provisions at 20' you will probably need a taller stem...