I will say, if you have never owned a shotgun or have hunted Ducks before, purchasing a shotgun with a 3.5" chamber and you load that shotgun with #2 3.5" Magnum, after three shots at a passing duck, especially in a pump action, you will have the hell kicked out of you. Hope you don't have false teeth, you would be looking for them in the water.
. I am just saying you don't need a 3.5" Magnum to shoot ducks, I don't think you need it to shoot geese either, but that is my opinion. We all used to shoot and kill them with 2.75" shells. The great thing about hunting and shooting in the USA, we have a plethora of options to choose from, sit back do so some research on what you want, and take your time, the season is over in TX this coming weekend, not sure where you are located, but you have time on your side. After the shotgun, now you have to consider Wader's, Decoys and calls to get you started. I purchased a pair of Wader's from Cabela's for 169.00, they are Cabela's branded and they performed great for me this season, also purchased a dozen decoys from Cabela's, the Northern flight brand and they worked well, what ducks we did see in TX, they landed on the pond with them. I would suggest, if you are serious, get a motion decoy as well. It can add up, but if you like it and enjoy eating wild duck, then you it will be well worth it. Have fun and let us know what you end up getting on your gun, enjoy.