Welcome to a wonderful journey. There's so much to be said in response to your post that this discussion will probably go on for an enjoyable while and probably consist of several threads. There's always more than one way to skin the cat and by starting now, I think Christmas will be an easy target.
Using "found wood" has its ups and downs. I have, more than once, spent much more in time and supplies, wear and tear, etc. by trying to make "free" material into what I am after. This is fine for me as I am primarily a hobbyist. Given your tools I would recommend going to the local lumber yard and ponying up the dough for the properly milled and dried material you want. Just decide where you want to spend your money because you will spend it.
You mention a planer as if it will flatten material and this is a common misconception. Planers make surfaces like the reference surface used, jointers make surfaces flat. A motorized hand planer is great for fitting a door on your house but, has little value in the jewelry box department. Sometimes a local cabinet shop will mill material for a fee. There is little that is so frustrating as trying to make a delicate little box with material that is out of square.
I'll go ahead and mention hand tools since someone undoubtedly will. A good jointer, smoother or other bench plane can help with flattening if you practice a bit. I'm a hybrid woodworker and while I have a till of hand tools, I generally rough mill my material on machines using the typical face joint, opposite face plane, edge joint, rip to width and crosscut to length method.
Back to materiel and striving for success; my local yard has a shorts bin that often has some interesting material in it suitable for smaller projects. The board-foot price is still the same but, I don't have to buy a 12 foot board to get what I am after. The smaller the project, the more forgiving the irregularities of the material. That is, a bow of 1/8" over 48" is not so problematic once I have cut an 8" length of this material off to make a part. This goes to your question of how to cut a 45* angle on a 6 foot board . . . the answer there is arguably a slider . . . seriously though, reduce your parts to rough, over-sized pieces before you start cutting things to finished size.
I'll let someone else chime in with more and once again, welcome aboard.
Last edited by glenn bradley; 06-20-2015 at 03:35 PM.
Any sufficiently advanced technology is indistinguishable from magic.
- Arthur C. Clarke