10k- 20k depends on what kind of accuracy you expect and what kind of ammo you shoot. I don't think you need an extra barrel if you shoot 500 rounds a year but if you believe in the upcoming zombie war then.....maybe LOL
Kind of depends on you financial situation too, if you have the money buy an upper and stick it away.
5.56, 6.8, 264LBC, SIX5.
The Nextgen of 308 rifles