I can tell you from experience that it is a good idea to go ahead and get an extra set of springs and detents for the lower receiver. Them small ones go flying with no chance of ever finding them, especially on carpet.
What are you thinking about building? Retro, A1, A2, A3, something for accuracy?
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As good as they are making them now, and with as many configs that are factory available, I would just find what I wanted and get it rather than trying to piece and part everything together. Check out Rock River Armory.