You will almost certainly need to replace the barrel before you get close to that 15-20k round count.
One thing I can recommend for a cleaning kit,
A small, strong magnet. The best way to keep little pins/springs from crawling off in the first place.
I keep several lower parts kits. The problem I've had the most if if I mangle a spring when I'm swapping out furniture. I recently bent the retaining pin on the bolt and I don't have any spares so I'll be adding those to my list.
Wouldn't an extra complete rifle give you all the spares you need? That's my thought process, anyway.
No offence meant but why disable a functioning rifle to fix a malfunctioning rife? IMHO strikes me as at best break even at worst loose/loose solution.
I'd rather always be able to have all my firearms up and running, especially now a days.
I posted the same exact question at AR15.com and here are the suggestions:
https://www.m4carbine.net/showthrea...R-manual-recommendation&p=2874639#post2874639
Oh my dear Lord. THIS right here. I've had spare hand guards magically turn into rifles.Keeping spare lower parts kits on hand can be problematic. I have it on good authority that they have a habit of turning themselves into full rifles...