Yes, as mentioned, it depends a bit on what you want. Decide what you want, and then decide if what you want can be purchased off the shelf or not.
My first AR: I bought a complete lower, and then ordered a complete upper the way I wanted it. Well, mostly the way I wanted it. There were budget limitations. But, I haven't changed the upper, so I must be happy with it.
My second AR: Built a stripped lower, and ordered the upper with everything I wanted on/in it. Was was going to be a simple KISS weapon evolved into a weapon loaded with more bells and whistles. I am very happy with how it turned out, but I spent maybe $100-$150 more than I had intended.
I was amazed at how easy assembling the lower was. I'd recommend it to all new AR enthusiasts. Read the how-to's a number of times before you actually sit down to do the assembly, take your time, and you'll do fine. As for tools, I think I only used a couple different size punches, a hammer once or twice, and a needle nose Vice Grip. Oh, and a couple Allen keys.
But buying is faster and easier. And since the AR is a modular platform, you can always change things down the road.
Pic just because;