Buy/build
Buy the upper of your choice. This way you don't need the vise blocks, barrel wrench and Go/No Go gauges. Doesn't make sense for a one time build. There are a ton of options out there.
Now the lower can also be bought already assembled. But if you are itching to get your hands dirty and say "I built it", they are really easy to put together. There is some tooling that is nice to have, but not really required. Plus, there are tons of videos on yt on step-by-step how to do it. I like Brownells and Midways.
As for home defense, 5.56 is much better with drywall than most pistol calibers which is why you see so many PDs switching to AR and away from MP5s. Plenty of articles on this.