Carl,
20' plates shouldn't be a problem. I would be careful in selecting a mill though, as most only saw boards, and their mills often don't cut square. I would bet a good percentage of them don't even have a square in their mill. As far as raising them, do a search on the forum for parbuckling.
I can imagine which Benson joint you're referring to, and I'm not a big fan. Nor am I fan of some of the joints in the Chappell book. Buy a copy of the Historic American Timber Joinery book from the Guild, and also check out Sobon's books. They're both essential reading.
There's really not a lot of good reasons to truck doug fir across the country, when you've got at least a dozen different species you could build with locally. You can even use a dozen different species in your little 16x20.
Unless you have very tall kneewalls, I think you can skip the ridge. You'd also be point loading the tie beams with 1/2 the roof load, so you'd have to beef up the tie beams a lot.
