Please forgive my ignorance here. But what are some necessary spare parts to keep around to keep around? Is it ok to buy a “kit” from one of the manufacturers? Or better to assemble a stash of your own with hand picked parts?

A spare complete BCG will cover you in most cases.

Having built several uppers, the small spare parts do seem to pile up a bit. Actually the extra uppers I have are essentially spare parts caches.

If you don't want to buy a complete BCG, some common parts that you might want to have onhand :

Spare bolt
gas rings
extractor and the spring/o=ring that go under it
firing pin

Most of the rest of the AR parts are fairly durable.

I have more parts than I really need but as a minimum, get a spare bolt assy or as cheap as they are now a spare bolt carrier group.

A buffer spring or two, a spring kit or an oops kit, I usually keep a Lower Parts Kit without the trigger or grip but they usually turn into a complete gun if I don’t watch it.

Odd things I’ve seen break, bolt release, gas key got worn and a receiver extension broken at the threads. Gas tubes get bent once in awhile.

Spare gun.

Then an oops kit.

The most common wear items we’ve seen break or cause problems are buffer springs, extractor springs, bolts and gas rings.

The spring issue can be resolved by using Sprinco from the beginning. An extra bolt in the grip solves the other.

If everything else is assembled properly with good materials they’re very reliable.