the armor's course taught me that GLOCK's don't break !
RSA's are a must, extra pins, pin push tool, extra mags, sights, the rear slide cover that has the "slot" cut in it would be ok to have, extractor ( I have heard of one case of the lip on it breaking).
hhhmmmm, IDK the only issue I have ever had with a GLOCK (and I have had several), I had an OE (plastic) front sight come off in the feild but there was no shame for it as it was original on a 13 year old G30.
GLOCK work mat, oil, cleaner, dry tooth brush, latex gloves, jewelers loupe mags mags and more mags and some ammo !
With only 33 parts you could throw a replacement of all from all calibers for pretty cheap, but there's not really a need.