I built one from the ground up, one thing about it you can say i put the whole bike toghter my self and not have to worry about some one saying hey isn't that so and so's old bike?
But if you can find a bike the way you are wanting one then go for it if you have the $ for it.
But myself i would rather build it, i have done 2 bike's this year from the ground up my girl's yfz and my banshee and it has been awesome doing it.
Good luck on what ever you decide to do you will be happy either way.