I personally wouldn't waste the money on a rifled choke, unless you have it to waste. I have heard pretty mixed reviews about them working well. I might try one if I could get one at my local walmart for 15 bucks or something, but I woudln't go out of my way to order one or pay twice that.

Just get some different brand and sizes of rifled slugs and shoot them through your gun to see what it likes. Try some remington sluggers and federal truballs. I have pretty good luck with winchester ammo myself.

Like any other gun you need to play around and see what it likes.

As far as how accurate will it be, that all depends on your gun and what ammo it likes, and how much you are willing to play to find the right load.
Also try different chokes, some of my guns like a modified choke instead of an I/C choke. Don't use a full choke though, that pretty much never works.

Another thing that will help is some sort of sights, I have pretty good luck with the clamp on fiber optic sights they sell. You will have trouble getting decent accuracy at longer ranges with a simple bead sight unless you have very good shooting form.

I have seen guns that would not shoot well past 50 yards and others that were quite acceptable at 100 yards. My mossberg 500 would get about 6 inch groups at 60 yards or so. My Browning A-5 stalker will shoot around 4 inches at 100 yards off the bench with 3 dollar winchester slugs. That's better than some rifled barrels shoot for some people.

My buddy has a Franchi 20 ga that he can shoot pretty well out to 90 yards with twin beads.

Really depends on you and the gun.

Paul