Yep, you're choice of gun and ammo is good. Most 12 gauges can take 2 3/4" or 3", either will do the job no problem.
You can buy used from a pawn shop who's an FFL, though they really vary in integrity, prices, etc. I would say you're better off buying new if you can afford it, a good shotgun can be found under $250.
Like any products, brands definitely vary in reliability, accuracy, pricing, but researching the specific model is more important since even great companies put out shit products from time to time. If you buy new, you're not too worried about reliability since even the worse American brands aren't going to malfunction out of the box (minus extremely rare instances).
Yeah if you can go to a range or outdoors and practice, then you'll want more ammo. Can't really tell you how much, depends how much you want to practice. Most common pumps carry about 5-8 shells at one time for reference.
