The 1100 has been around for 49 years, been made in several different configurations, and has been used successfully to do everything. $400 in the condition described with 2 barrels ain't bad. I still have my '63 version, and several others, and love them. And I duck hunt.
But, now days most people use at least 3" chambered guns for waterfowl because of the non-toxic shot requirements and the desire to shoot steel because it is the cheapest of those. A 2-3/4" version 1100 - which I suspect the one you are looking at is - is not that versatile.
You obviously have a lot to learn; we all did once and it's a fun trip so don't worry. A good pump gun with a 3" chamber is not a bad place to start.