Hello,
I purchased Guild Wars: Nightfall about a year ago with similar intentions and I just would like to offer my two cents on your situation. Guild Wars has a level limit of 20, and if you play about an hour or so a day you can easily reach it in maybe 2 or 3 weeks. The gameplay is heavily focused on PvP matches which are 4v4 and are set in deathmatch style arenas. I found the biggest downside of the game to be the fact that to unlock all of the available skills for each class you have to buy all 3 GW versions...each one includes a couple more quests but doesn't raise the level limit, unfortunately.
I got tired of the game rather quickly because once you reach the maximum level (about halfway through the storyline) the rest of the game just becomes a trial-and-error-combine-your-skills-to-progress type ordeal, in which you spend all of your time fiddling with you last few attribute points in order to maximize your skill effectiveness given the limited character level you can reach.
I know this was a bit off topic but I wouldn't recommend the game to someone looking for a WoW replacement. There just isn't enough content in the game to keep you interested for long. The game does look fantastic, but unless you really really really like PvP, you won't be playing this for over 2 months.