Double Star and Tactical Machine lower receivers for the AR-15 Rifle are made from 7075 T6 Aluminium.

The key to any good project is the foundation and on an AR-15, that is the lower receiver.
Think of it like a house. You can build the most incredible house imaginable, but you have to have a good foundation.
The lower starts with the lower receiver. Double Star and Tactical Machine makes a stripped lower receiver that are readily affordable and are of great quality.
The DoubleStar Lower Receiver Parts Kit contains all the smaller parts you will need, including the trigger and grip for $69.00.  You will also need a buttstock and related parts.
I think that an adjustable buttstock is the only way to go for a home-defense carbine. The DoubleStar AR-15/M-16/M4 Six Position Adjustable Buttstock, which comes with everything you need.