Guild Wars is comprised of three campaigns and one expansion. The three campaigns are Prophecies, Factions, and Nightfall. These are all stand-alone games, each with its own unique storyline. Eye of the North is the only true expansion. You must have a campaign to play Eye of the North. If you have multiple games linked to your account, your characters can travel to campaigns other than the one in which they were created (Assuming you complete the appropriate quests).
Each campaign, on its own, will have four character slots. Every time you add a campaign to your account, that you didn't have before, you get an additional two character slots. So, you get eight character slots for linking all campaigns to one account. Eye of the North doesn't add character slots.
As for the cost, it shouldn't be too high. You should be able to pick up Guild Wars: Trilogy (Prophecies, Factions, and Nightfall) for about $50. Eye of the North should be about $30. It probably goes without saying, but never buy used.
The developer of Guild Wars, Arenanet, has stated that they won't be shutting down the servers when Guild Wars 2 goes live, which will be awhile from now. If you decide to get it, you should be able to play for quite a few years. Overall, I'd say it's a good buy, but don't expect any great developer updates.
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