Dont worry, what will come next wont be anything like Fallout 3
In the beginning things look alot like FO3 but that changes rapidly
you will LOVE the quest at Repconn
Silly question, if you kill something, is it dead for ever or does is spawn back when you visit the same area again.
Depends. If you kill a NPC wich talks to you, then it wont respawn. Things like raiders, fiends, Soldiers of faction appear randomly.
A cazador nest for example, will respawn new cazadors after a while. But a random encounter will respawn diffrent things. But all of this takes time (ingame time that is)