I had bought the spring kit and the pin kit early on as the foundation of my repair parts kit. From there I added extra individual bits as needed when ordering other things. It's a good idea to have spares of everything, and now that I have 7 ARs, I always keep several of everything in a large clear plastic nut/bolt box. Bushmaster is fine stuff, haven't had any issues with their springs, triggers, pins. I have also ordered RRA and DPMS pins/springs too and they work fine also. Don't forget a spare buffer and buffer spring too if you want a good repair/maintenance kit.
As for bolts I do prefer MP CMT and LMT bolts. And I always keep a spare BCG in my repair kit. However, if I ever have a bolt failure, and use the spare, that spare will then stay with that gun and I will order a new spare.