The Remington 870 Express is pretty entry-level and is a bargain for your purpose if you buy it with the two barrel set, a shot barrel and a rifled slug barrel. Definitely cheaper than buying an accessory barrel later.
But other brands offer the same sort of deal, including Mossberg. I have their 535 and like it better than the 870 Express with a slug barrel because I like the safety better (on top for your thumb instead of a button on the side of the trigger guard, too far to the rear to be convenient. Get to a gun shop if you can and handle a few different shotguns and you might find little things like that are much more to your liking on one gun than on another.
Bear in mind that these "entry level" guns can last a lifetime and may never give you an excuse to buy another shotgun.