1. sure there are powders that will work for both, and depending on your guns and bullet weights, one might be ideal for both, but why limit yourself over $25? A lb of powder is cheap. so dont do it to save money. different powers work better for specific bullet weights in each caliber. so if you want to shoot heavy .223 bullets and light .308 bullets, one powder isnt ideal. It will work, but you will be compromising on one caliber.... for $25, I would go for the best in both. If one powder happens to work well for both, great, but dont do it to save money, do it for performance. On the other hand, it also depends on what you are doing with your guns. If you are hunting with .308 and plinking with ,223, you would obviously want to choose a GREAT .308 powder that also works with .223. If its the other way around, you want a GREAT .223 powder that also works with .308.
2. any manual will work, I use mostly hornady bullets, so i bought the hornady manual (its also the most recently published currently) but I am not limited to hornady bullets. If I want to use another brand of bullet, I just start low and work up, like always..... I also have a few of those "one book one caliber" paperback dealies for backup.
3. Dents. If they are smooth, use them and it will iron out when you shoot, if there is any kind of crease or corner, toss them.