In my experience, most of the wear and tear that a rifle receives comes from getting in and out of vehicles, or transporting them in one. In the field, my rifle is in its element and is in no danger of getting beat up. I do not hesitate to take any of my rifles into the field regardless of their worth in dollars/euros. I have no "safe-queens". In fact, I find that a rifle that has not been taken out and shot for a period of time gets poutie, and doesn't shoot as well as when I take it out more frequently.
Regarding powder, don't hesitate to use your 3Fg powder in anything you have. One of our local shooters only owns a 20 gauge smoothbore (Chambers Pennsylvania fowler), and he shoots a .600" ball, heavy cotton patch and 85 gr. 3Fg GOEX all day long...he is very difficult to beat on the trail shoots.
Regarding wood species, Mr. Kibler will not sell you a piece of wood that is inappropriate for the gun you're building. I have no way of knowing how you handle your rifle in the field, but I doubt you mistreat it, so don't be afraid of getting a guns stock that won't be tough enough for you.
Calibre: a .50 is a good compromise for your needs and you will enjoy shooting round balls with it, and it is still very representative of what was used in the period of history we are emulating.