... For a lower, I would go with the cheapest stripped lower you can find... brand doesn't matter. There are Surplus Ammo & Arms lowers for $80 online (plus shipping and FFL fee), or at least around here you can find them at a LGS for around $100 or not too much more.
For the upper, I would go with PSA because they have great prices, and meet or exceed the mil specs on everything. While that isn't strictly necessary, especially since you said it isn't going to see a high round count, the extra quality assurance is a good thing. HD is a pretty serious role, after all. Their chrome lined barrels in particular are very nice, being made by FN.
I would go with one of their "fall madness" uppers while they're on sale. I would go with a 16" barrel for your stated role, since you mentioned home defense, and you didn't mention a need for any longer range work. I would go with a mid-length gas system since they have the optimal dwell time for a 16" barrel, give you a little more sight radius and handguard length, and are easier on recoil and on the internal parts. For barrel profile, I would go with the lightest you can get, for ease of handling in tight quarters and for being nice to carry around the farm. I would prefer the pencil profile, but as of right now it looks like they aren't in stock. The mid-length light profile premium madness looks nice too, and is a great price at $279.99. I actually just ordered one of those myself for my Dad's AR build.
Get one of their discounted premium classic lower build kits to finish off the lower, one of their M-16 BCGs, and their discounted mil spec charging handles, and a Magpul MBUS rear sight and you are in business. Spend the rest of your budget on a good red dot sight, a flashlight + mount, and maybe some different handguards to mount the light on.