Build your first one to specs. It's amazing how good they can be. Do some research and find a quality kit. Learn it inside and out before you modify it.

Buy the armourer's guide and the barrel tool. Bushmaster had some good parts and tools years ago. Some of the Colt parts are a little different but you probably won't see those anyway.

I haven't built one for 15 years but it's easy if you take the time to learn as you go. Look at how it works and make sure springs don't bind, etc.

I recently thought about buying some lowers and didn't. Receivers were well under a hundred bucks before TSHTF.

Read about barrel twist and bullet weight. Wouldn't hurt to have some stripper clips, magazines and ammo even if you don't find a deal on parts. The gun outlasts the gunner.