Look at what is critical to operation and what takes a beating or heavy use:
- firing pin
- bolt, lugs can break off but more likely the extractor, extractor spring, ejector spring
- bolt cam
- bolt o-rings
- hammer spring
- trigger group
- buffer spring
- buffer retaining pin
Then I would look at what could easily get lost during field or detail strip:
Pretty much every pin and spring, and the cotter pin that secures the firing pin.
To make it easy, maybe a lower parts kit and a spare complete bolt. Might want to add a gas tube. The bolt would be my top priority or a bolt rebuild kit.
Sure I missed a few things but that is my first thought.