I've heard that Leviathan is a lowly populated server but I see tons of people on whenever I'm on and at all hours of the day/night.....I would highly recommend finding a Free Company(FC) and/or Linkshell(LS) to play with, they will help you along your way in the game. If you need a friend look me up and check out the server forum on here for a FC or LS

as for the DPS question it depends on your style of play, I like ranged DPS so I play with a Black Mage but am trying out all the other classes also to try them out