Lots of good advice in here. An O/U is not ideal if you want to use a scope, IMHO, for the reasons mentioned.
Of all the shotguns I've put slugs through, two stood out for accuracy: A Winchester 1300 and a Baikal MP153. Both shot slugs as accurately as many deer rifles shoot bullets. The other shotguns mentioned here (particularly Mossberg 500 and Rem 870) have at least two generations of reliability behind them. I think you could also likely find a Remington 1100 in the same price bracket. I suspect that, in the long term, all these guns would be cheaper than a low-end double barrel, as they're easier to fix.
If I was in your spot, I'd first find out if slugs were legal, or only buckshot. Then I'd buy a pump action shotgun with a 24-inch barrel, if you could find one. Look into a turkey hunting shotgun. Shorter barrels = more noise and recoil = more flinch = less accuracy. If you're limited to buckshot, a bead sight is fine. If you're shooting over 50 yards with slugs, you want iron sights. Under 50 yards, a bead sight should be fine for slugs too, as long as it hits where you put the bead. If not, well, you'll need sights or a red dot scope.