Model 12s are very well built guns. But if you want to duck hunt, you are going to have to use non-toxic shot. This gives you two options with that model 12:
1) You will have to buy a special softer shot. Steel shot, and most high-density non-tox loads (which are required by law) are VERY hard. Much harder than lead. They will (over the course of several shots) do perminant damage to the barrel of that gun. Moreover, because that full choke is very constrictive, you likely won't get very good patterns with it.
That said, you can purchase softer non-tox loads such as Kent Tungsten-Matrix or Tundra. Environmetal makes a "Classic Doubles" load which is designed for older guns like that model 12. Only problem with these is they are not for cash strapped hunters. On average, you will be spending $35-40 for a box of 10 shells. That's $3.50-$4 every time you pull the trigger.
2) You can spend another $250 and have a gunsmith put interchangeable choke tubes in the barrel. While this will make the gun much more versitale, it will negatively effect the value. Once you have those choke tubes installed, you will be able to safely shoot any type of shot, including economincally priced steel shot which is often around 50-75 cents a round instead of $4.
I guess you do always have the option of buying something else used too.