I agree that your first no kidding gun purchase should be a concealable handgun in a defensive type caliber 9mm or better power wise. There is a long laundry list of reasons why I think that should be your first aquisition. So I think your on the right track there.
That 16 gauge is a fine gun but its a lot like a 28 guage...ammo is a bit uncommon and hard to come by. Additionally unless you hand load your not likely to find a lot of the loads your going to want for defense like you will a 12 gauge or 20 gauge, such as 00 buck, 4 Buck and slugs. Im sure there are some being made but your going to be looking long and hard for them and your going to pay dearly when you do find them. That being said I think you would be money and time ahead looking for a good 12 gauge. You can just about buy commercial loaded ammo for that about as cheap as I can reload for it, really! Even these days during the ammo drought of 2012/13 you can still usually go down to wally world and pick a few boxes at standard prices and not get gouged in the wallet. Around August/Septimber when Dove season kicks in at least here in Texas you can buy the birdshot loads by the cases on sale for less than 5 bucks a 25 round box!!! I would suggest that should you be considering reloading, that you purchase the same brand so that you get the same hulls and only have to use one wad to minimize components on hand.
As for shotguns, since you on a budget, shop around. I have picked up some great used Remington 870 pumps at pawn shops for a 100 bucks, although most were a bit more. I just picked up last season a Mossberg 835 Turkey gun with 5 shot capacity, 22 inch barrel, camo coated, fiber optic rifle sights for 219.00 out the door brand new in box! Thats right there with the Mavrick shotguns but with a lot more options, whistles and bells. Deals are out there so be patience and keep your ear to the ground and you can likely stumble into a great deal too. Half the guns in my collection and I wont say that number here but its a lot more than most people will ever own were purchased for a song and dance compared to the going rates for them so save your pennies and keep an eye out and you too can scam on a hell of a deal too.