1. 3 standalone games, 1 expansion. I'll go out on a limb and say most are around $20-$30 now? Pretty soon you can get the trilogy pack that has Prophecies, Nightfall and Factions for $50.

2. No, with Factions, 1-2 days.

3. It's worth it. Mostly for PvP, but there's a lot of missions PvE-wise as well. I played from the week it came out until about 3 months ago, so it's lasting.

4. Depends. Random Arena; you and 3 others randomly assigned to fight another team of 4. Team Arena; same as random, but you make custom teams of 4. Alliance Battle; 3 groups of 4 in a node-control game, most fun imo. Guild Battle; 2 guilds in groups of 8 fighting to kill the other person's Guild Lord. Hall of Heroes; 3 groups fight to capture the Hall of Heroes.

5. DPS, I'd say elementalist, they put out the huge numbers. Same with assassin.

Hope that helps.