I call this an LCK. Lower Completetion Kit.
I'm working on building a lower right now, and was just consulting a buddie who is doing the same.
We both have our choice trigger and grip, so I had complied this list for us to order what else that we need.
If you want to get tricky. You can sub the Detent Springs with flint springs from a Mini BIC, and the Safety spring with a flint spring from a normal BIC but cut in half, then the other half you can use on a front sight.
Oh and of course, you're still going to need a stock and tube, but those are usually not included with LPKs.