Re: Spare Parts kit?
In order of importance:
Bolt (which will cover your extractor, ext. spring assembly, ejector/spring and gas rings) This and the firing pin will take you a long way without taking up much space or $$$.
Firing pin
Other Small bolt parts (Cam pin, firing pin retaining pin)
Carrier assembly (or at least gas key parts)
Trigger/Hammer springs
Trigger/Hammer
Piston/pison parts (should you have not have a gas gun)
AR parts aside, you will better served with spare batteries for optics/lights (should you have them) and some lube in most cases.
Stop and consider all of the things that are superfluous on the AR platform. No sense in stocking up on ejection port covers and forward assists. If it isn't involved in the firing/cycling process, don't worry about it.
For my "work gun" I have a complete bolt, firing pin, and piston packed with my gear.
As my colleague mentioned above, having a spare gun is not a bad idea if you can afford it. Personally, I will have a go-to tricked out version, and a bare-bones, KISS version as a spare (or to brush up on the fundamentals if I want to run only irons and no lights).