I'm doing something similar I have a idea I'm planning to make into a trilogy., I'm starting with one set of characters and have the first part end taking off with another Character.
Style of writing use one that fits you myself I'm detailed and usually focus on characters emotions and thoughts rather then what is going on around but that is my style find your own you love and flaunt it.
Be it Detailed story telling, First person Pov or just something new find what clicks.
As for buildup throw in some curves when telling a story make a chapter that seems normal end with a bang do something people just didn't see coming and of course never put all your cards down on the table., tell a story without making it guessable as to where it's going and always keep the best for last plot wise.
Last with the bringing characters in problem write it in the story once you've weaved in enough plot in the tale to do so I have many ideas I just want to write about now but know it's not time., write till it feels right to do so even if it means writing something you don't really have the umph for it's all part of the tangled web that is fiction.
That is all I have for pointers if you can or have a draft of this tale to read I could get inside it's head and maybe help more detailed at fixing or adding plot aka meat to the tale lol.