If I might add, I have been hunting deer for over 40 years and up until a few years back (when I bought a muzzle loader) I always used a Mossberg 500. For the money its about the best you can buy, hands down. Not saying a Benelli isn't a great shotgun, but for a beginner a Mossy is a great gun. I have several barrels for mine. A smoothbore 24" slugster barrel with rifled sights (deer), a 24" rifled camo barrel again with rifled sights (deer), a 28" modified barrel (pheasant and other small game) and a longer full choke barrel both with glow lite sights. These guns can quite often be found used for a very reasonable price. A friend just sold his to a young man like yourself for $125 with an extra barrel although you may not be able to find such a great deal, and maybe you might find an even better deal. I have hunted deer, rabbit, ruffed grouse, pheasants, timber doodles, quail, ducks and goose with mine. I've had it for 40 yrs now with no problems whatsoever.
If you buy one used, any brand, I would advise shooting it before buying it unless buying it from a gunshop where they will stand behind it if it has a problem.
A scope is great, but with the distances you normally shoot deer at you can do well without one. Let me warn you though, once you shoot through one its hard going back to open sights....lol. At least it was for me.
And the advice to buy a clay pigeon thrower is a great one. Practice makes perfect.
Best of luck, be safe and have fun
oops, didn't see you already made your choice. An 870 is a great gun! No go have some fun....!and I hope you have great success.