I always promote home-building over store bought. You can save a ton of quiche, pick the exact parts you want, and have the joy of putting it together yourself.
Although admittedly I've stuck to completed uppers lately, just because I hate dealing with barrel nuts. And Aero has so many damn configurations now, that it's pretty easy to find the exact setup you want.
If you find an upper you love, then all that's really left is cherry picking your lower, LPK, grip, FCG, BCG, foregrip (if desired), charging handle, sights/red dot, buffer, buffer tube, end plate, and stock. Even the most unskilled noob builder can put together the primary parts of a lower in about an hour. Maybe bump it up to 90 minutes if you really take your time.
As for tools all you really need is an AR-15 armorers wrench (mostly for the castle nut), a set of quality punches, a ball-peen hammer or jewelers mallet, some masking tape (so you don't mar your build), and you're good to go. A set of hex wrenches and some loctite might come in handy as well.
This video is a must-have bookmark for any home-builder. It slowly walks you through each and every step, and has saved my hide a few times when I've forgotten something. I'll be honest, I probably spend more time on that stupid pivot pin retainer and spring than any other part of the build, because I'm stubborn and refuse to buy "the specific tool"...
EDIT: And if you buy a completed upper, your work is done at the 24:00 mark in the video.
