Recovered bullets are one of my main sources of lead for casting. My partner and I smelted down a couple hundred pounds into ingots just the other day.
Once the lead becomes molten, the jackets, dirt and debris will float to the top and you can skim it off. If you go to the thrift store and look for one of those large slotted spoons they use for boiled vegetables, it will be ideal for skimming, since it lets the molten lead flow through the slots or holes, depending on what you can find.
Wheelweights will give you some tin and antimony, but if you can find some 50/50 solder at a reasonable price, that will help, too. The Lyman Cast Bullet Manual would make a good investment for you, since it has all the information you would need.
It seems that some plated bullets are tougher than others, since we're finding more and more of them that are still intact after firing. Those have to be cut open some way for the lead to get out. If your department is using them, you'll find that out. It also depends on the material your berms are constructed of. If there is quite a bit of rock in it, the bullets will break up better. Our berms are sand, which makes a difference.
Hope this helps.
Fred