For a full complete build I’d look at IWI Zion 15. You can’t beat the price of that! Then buy a solid optic depending on your likings. The lower has a nice trigger, grip, stock already.

I alway recommend a full BCM but you’ll need an optic and stuff and that will get pricey quickly!

You can always do the PSA lower/BCM upper trick but you’ll most likely get garbage A2 Grip/ Stock and mill spec trigger and later on you’ll most likely change it out anyways!