The lowest price on AR lowers is this.
AIM Superior Arms S-15 AR15 .223/5.56 Lower Receiver Superior Arms S-15 AR15 .223/5.56 Lower Receiver
You will need to get it transferred to your local FFL (gun dealer).
When you get the lower you will need an AR15 Lower Parts Kit right now they are hard to find (I would suggest Model 1). But when you get these and a stock, you will be ready to assemble. Here is where to start, click this link & then click the AR-15 Stripped Lower Assembly Video.
The Range Bag - Life, Liberty, Etc.
When you get the lower assembled all you need is a upper w/ bolt & carrier (again I would suggest Model 1) and your ready to go to the range.
I have yet to build a 1911 so I can't help you there.
Good Luck & Happy Building.