OK. Forget the shorty barrel, its useless except for home defence. Go get a 26" or 28" barrel.
The Remington 870 is a very good gun, and very cheap at that. It will last you a long time, and parts are very plentiful.
I would forget about the rifle for the time being. A rifle of a caliber larger than .22 is damn near impossible to go out and plink with, regardless of how cheap the ammo is. It is simply to powerful--and thus usually too dangerous to go shooting cans with. Unless you have a good reason to own such a gun, move on.
And as for shotgun ammo, you wont be shooting slugs or buckshot unless you are hunting. A box of 25 shells of birdshot, the stuff you would shoot to go skeet shooting or general goofing around, is about $3.50. Not bad at all.
A shotgun is (next to a little .22) easily the most fun you can have with a firearm. I guarantee you, unless you live in an area with *lots* of free land that you can shoot a high powered rifle on, a shotgun will get *much* more use--and therefore more fun
There are skeet clubs all over the place, and if you cant find one within a 30 minute drive, a small field, a $5 hand thrower, and a $4 crate of skeet is more than enough entertainment for a few hours.