The PSA kits fit that lower really well,I may be wrong but I can't imagine the pins and springs being much different from brand to brand ,most likely all are made at same factory.Triggers are probably the exception.As far as BCG,I would go with milspec and get it included with upper assembly that way you shouldn't have any head space issues.I have seen stainless spring kits that cost more than standard,but not sure if they would be worth exra cost,biggest thing would be what you plan for rifle,keeping forever or possible get tired of it and trade,if keeping buy best parts available if possibly trading just buy standard parts otherwise you will never recoup what you put into it.JMO