1) It depends on how hot and dirty you let the barrel get. Some say that pushing 40 Gr bullets fast (3900+fps) wears the barrel faster than, say 55 Gr bullets (perhaps 3650 fps). You could get primo accuracy until 2000-ish rounds. Then again, you might as well bite the bullet (so to speak) and just consider barrels like tires...wear items to be replaced. Let the barrel grow cherry red hot, and expect the barrel to shoot well 1200-1500.
2) 1-12"? 60 Gr V-Max, but try 50-55 Vmax and 50 Gr Nosler Ballistic tips (or Sierra Blitzkings) too. 60 Gr VMax might be one of the larger varmint bullets well suited for that twist.
3) Factory, dunno. .223 factory ammo price has gone up dramatically lately, I don'e know where they landed (Check Midway/Natchez etc. current prices). I reload, so it's not much at all...the incremental amount of powder...10-ish grains. The bullets are the same size, and the initial brass isn't that much more for .22-250
4) For range and trajectory, .22-250. .223 Rem has one very nice trait: The recoil is just low enough that you see hits through the scope. .22-250 has more reach and power. .223 Rem generates less heat in the barrel than .22-250.