Yes, ARs come in many calibers now, but a lot of them are exotic. 5.56 in a SHTF case is good just because of abundace, but I wouldn't expect to find a cache of 300 blackout, 300 Whisper, 6.8 SPC, 30 AR, 6.5 Grendal, 450 Bushmaster, 50 Beowolf, 458 Socom, etc. In fact, I'd view that as a negative just because other than 5.56 I wouldn't expect to find any ammo. So you could buy an upper and hope you'll find ammo, or you could use that money and get something you'll know you'll find ammo for.
There is a lot of 7.62x39 out there, which is a major plus for the AK. And prices on ammo right now isn't bad so getting a thousand or two thousand rounds isn't out of the question.
Believe me, I know about meager wages, but I also learned long ago that quality costs money, and quality usually holds its value well. When I bought my Dillon 550, or any of my guns, even knives, it hurts. The thought well I could buy a new EDC folder, or I could make 2 months' car payments, crosses my mind a lot. But I do have peace of mind that if I have to sell it I can get a good chunk back. Firearms are money pits, always have been, always will be. But quality is still quality and people pay for quality, new or used.
When I said shtf weapon, I was not meaning just cliber changes, though that's good to.
I was thinking something short like sbr for moving around and a longer accurate upper with a scope for hunting and long range duty. 9mm or 45 acp as a suppresor host upper. Heck a 22 adapter is also nice to have in the bag. The ak is awesome, but not as versitile.