There is nothing wrong with buying one already built. You do want to match the twist rate with the bullet grain weight you are planning on shooting and the grain weight also effects velocity as that is a 1:7 for mainly a heavy grain bullet. If you want to build, build it, they are not that hard if you buy a complete upper and just buy the other parts and build a lower. Some uppers come with bolt carrier and charging handle and some do not so pay attention. And you will need to match optics with what you are trying to accomplish too.