1.) The other games are not expansions; they are standalone games as well. You do not need to buy any other games, however having only one campaign will severely limit your capabilities in PvP.
2.) Depending on which campaign you buy, you will reach level 20 at different times. For example, the first campaign, Prophecies, levels its characters rather slowly. However, in Factions, you can get to level 20 very quickly.
3.) Guild Wars is definitely worth the money, being a single pay game. It has a great storyline, awesome quests, no grind, great cut scenes, and amazing PvP.
4.) The PvP system is great in Guild Wars. You can have alliance battles, fighting over checkpoints on a large map with people from your enemy faction, which is 12v12. You can do random arenas, which randomly places you with a team of 3 other people against another random team of 4, team battles, where you get to mix and match, and heroes ascent, guild battles, tournament battles, as well as a lot of other things I can't remember right now, which are still AWESOME, and yield great prizes.
5.) The beauty of Guild Wars is that classes are not restricted, by any means, to certain roles. For example, there could be an elementalist that acts as a tank instead of a DPS character, or a warrior that deals damage using spells. At any rate, a good DPS dealer would be an assassin, or an elementalist. Warriors too. Go necromancer for the warlock sort of DOT damage.