When assembling a stripped lower receiver into a complete lower ready for an upper receiver, there are innumerable options. Different parts kits, adjustable trigger-and-hammer sets, all sorts of pistol grips, etc. are available for the choosing. When deciding how to shoulder the completed rifle, the choice comes down to a fixed stock or one with an adjustable length-of-pull.
Should you opt for a collapsible stock, the ATI TactLite Six Position Mil-Spec Stock comes complete with buffer tube, buffer spring, buffer, castle nut and lock ring to complete your lower receiver build. The stock itself weighs less than 6 ounces and installs in seconds (obviously longer if this is part of a lower receiver build).