trying to truly cut a tapered barrel square requires a decent jig and tools.
I dunno, I've cut alot of tapered barrels on handguns, rifles and shotguns without jigs. But then, I've got alot of metalworking experience and a good eye for geometry.
Younggun-
There's really no way for us to tell you what kind of accuracy to expect. Some guns shoot rifled slugs really well, others not so much.
As far as picking up something else on a budget, it's totally doable. I see old Remington model 10's for $80-$150 all the time.
Just make sure it has a 2-3/4" chamber. 31's turn up cheap, too, and are a more robust shotgun. The Pedersen-designed 10 is a work of art, but a little more delicate than the Winchester 97, 12 and later Remington models.
Heck, another good one that turns up cheap is the old Wards Western field pump (made by Savage). Usually no more than $150, they have one seriously solid (and distinctive) receiver:
Just keep an eye out on the used shelves, and do your homework. Lots of great older shotguns out there for excellent prices if you know what to look for. And like I said,
make sure they're 2-3/4" chambered. A 2-1/2", 2-9/16" or 2-2/3" gun will chamber and fire 2-3/4" shells, but it could cause a dangerous pressure condition as the hull expands into the forcing cone. Without a gauge, the best way to check is to remove the barrel and see if a loaded 3-1/2" shell will go almost all the way in. The guys behind the counter should (hopefully) be able to help you with that (and the disassembly of a used gun; I wouldn't just start taking one apart in the showroom without permission. Bad etiquette)