As long as the ammo was properly stored it is safe to shoot. Not super accurate but good enough for government work! If the mags are like new or even good condition they would be worth more to the right person.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek