A complete bolt should be in every AR owners range bag. After that, a lot of it depends on the specifics of your build.
Bolt carrier with a new cam and firing pin would be good parts to add after the bolt.
I would probably go with a gasblock and gas tube next.
Next a charging handle.
Then a complete set of springs.
Then a bolt rebuild kit.
I don't use buffer retainers or forward assists at all. They are prone to breakage so I just strip them out if I end up with a lower that has those parts and never buy them when I build new.