Resale will be difficult for an off the shelf chambered upper. I could see where you are concerned with the 556 ability to kill but a good varmint round should solve most of those issues. 6.5 Grendel is what i would've done. Amazing ballistics for long range and will handle medium to large game, and is becoming commercially available now.

At any rate, building them is half the fun. As long as you have a reloader... no problem.