A) as was said, broken steel is the one that changes the ending. i don't think you miss anything as the ending is kind of weak. and i don't think it's possible with the dlc installed. if you want to see it, just youtube one to get an idea of what you missed after you play it. really isn't anything to write home about.
B) broken steel and point lookout are the must buys. the rest can be skipped, though OA has some good gear. and mothership i think has some good gear - been avoiding that so far in my game, so can say for sure.
C) have the pc version, so not sure, sorry.