Let me make a suggestion. Don't get paralyzed by analyzing. Just find what you can and start building!

If you need to scarf short pieces into long pieces just do it. I wandered around the lumber yard looking at everything trying to make up my mind last week. Just couldn't seem to make a choice. Finally just settled on the "spruce" the yard has as my best option.

I went home and rebuilt the scarfing jig (see other post). The started ripping and cutting scarfs. Within an hour and a half I had the stringers glued up and clamped. Next time I will not spend so much time worrying about the wood. If they have WRC that I can use I will just go with it. If it is to knotty I will just go to the pine section and use that. Scarfing up the stringer is no big deal.