Just a few things to add to the wealth of information above:
1. When selecting boards, don't pick ones that were cut right from the center of the tree. Those love to warp, twist, or even split. Boards with the end-grain parallel to the short edge are most stable (of course, those are just the ones you won't find in the cheap grades!) Also stay away from boards that are significantly heavier than their fellows. And leave behind any where the grain on the face of the board runs diagonally.
2. Definitely let your lumber acclimate to layout space for some time (weeks, preferably) before cutting it.
3. I think you will be fine with L-girders made from a 1x4 on edge with a 1.75" flange. All the flange is for is to keep the 1x4 from bending sideways when a load is put on it. OTOH, I find that ripping lumber is miserable. It might just be miserable enough to spend extra to avoid it. Some places you can get 1x3s to use as flanges.
4. The layout at my parents' has been in place for some 17 years. It is 30 x 17, and uses 1/2" OSB on top, and 2x2 legs, with diagonal bracing. We have had no problems with either, though we don't climb on it. It is in a basement, too.
FWIW!
