Re: Number of cases..
One option nobody mentioned is just buying .308 Norma mag brass. Norma makes it, Midway carries it in 20s and 100s for a hair over a buck a case which ain't exactly free but it isn't too bad either. At least you're not shooting .338 Lapua, xx-.378, or a Lazzeroni case. Usually Norma brass is pretty fair quality.
Have you built the gun yet? If not, you might also consider .30-.338. Hint: If you're in a neck turning mood, neck down .338 brass, if you're feeling lazy, neck up 7mm mag brass.
Whatever you do, I suggest getting 50-100 cases more than you think you need, prep them like the rest, then set 'em aside for a rainy day. That cache can keep you hunting if brass gets scarce somewhere down the road.
Flex is for people who don't have lockers.