There is a an official Guild Wars thread in the netplay forum, I'll try to find it but I can answer some of your questions.
When starting at level 1, you can start at any campaign you have activated on your GW account(have bought). If you have nothing else to spend the money on, or really like Guild Wars, go ahead and buy all three (although in my opinion, factions is pretty bad. Like, I really didn't enjoy assassin/ritualist or the story/areas much).
No the expansions are not necessary unless you want to, you know, play them. They don't really add anything but story and classes (if you buy an expansion I think it comes with all the base classes like warrior, mesmer, ranger
I think).
Edit: Here is the official GW thread
http://www.gamingw.net/forums/index.php?topic=8286.0