RE: getting started
Get the 870 supermag in black synthetinc (i have killed turkeys at less than five yards with that gun, the camo doesnt matter) then you will be able to shoot 3 1/2's tho you will never really need to for waterfowl unless your going to be pass shooting. If you watch your local gander mountain adds you should be able to find one for around 300 with not much trouble(i got mine new for 289). They dont come with multiple chokes(come with a modified that can shoot both steel and lead) but a modified id the only choke i have ever used for waterfowl(dont want too tight of a patttern) and i have killed my fair share of both ducks and geese.
A Nova for 419 is most likely used too, and you never can tell what your getting in a used gun.
One more thing, ask how many benelli guys would drop there gun in the water in mid december, let the water freeze, and then pull the trigger and have to go go BANG.Mine and wdog's(my hunting partner) 870's have been through hell and back more than once, and that guns still fires just as good as it did the day i bought it.
Hope this helps your decision.
BTW, where are you from?