You can purchase Berserker right after you finish the battle at Helm's Deep. He is located near Helm's Deep just look for his icon on the map.
The Mithril Fireworks can not be obtained until AFTER you finish the all of the levels in story mode. Once you finish the game and unlock Free Play for Middle-Earth you can take Legolas (or any elf, or tall person wearing spring boots) to Mount Doom, the black smith plans are above the entrance to Barad-dûr.
"Man how to people figure this stuff out? A rubber band on the R button." - HYPERMECHA
So all the mithril blockages one finds on the middle-earth map can't be destroyed until after the main storyline is complete then? since as far as I can tell, 'bought' characters can only be used within the missions...