The part I've had to replace twice now on my G17 is the striker. I think I have just been extraordinarily unlucky, because this is not typically considered a high wear part. Parts I would keep would be springs (recoil spring, trigger spring, slide stop) and extractors. It might be good to have a spare striker assembly too.
They are incredibly easy to work on. No fitting or machining, entirely modular. There are prettier pistols, there are pistols that subjectively feel better in the hand, but there are none I would rather have in a Mad Max scenario.
Prices at the Glock store for factory parts seem reasonable -
Recoil spring $16.99
Trigger housing w/ejector $9.99
Trigger spring (looks standard, no Gen4 difference) $2.09
Slide stop $9.99
Extractor $18.19
For the "stryker assembly" I am guessing you have to get the firing pin and all the other components (like around $67 for all the parts) as I do not see an "assembly" package.*
*Correction, I see a striker & spring assembly for $94.99, I must have been looking at something wrong earlier.
Also, I am wondering if mag springs might not be good to get as well? Just $6.29 for those.
Ooh, ported barrel! $235. Hmm, maybe later.
http://www.glockstore.com/glock-factory-parts