You have gotten great advice already, so not much to add. I would say that for me, and it might just be me, the real value of the "chaps" is that my pants stay dryer and last longer. I typically go through a pair of the Filson ones every 2-3 seasons hunting between Nebraska / Iowa, mostly they get torn up along the hems. So that being said, the chaps are helpful to stay dry (need to dress them), but if you are not hunting a ton, probably won't matter as far as the wear on your pants. As far as a gun goes, shoot what you are used to using (unless you want an excuse to buy something new!) Whatever fits you well will work.
Along the lines of "don't shoot the guides dog..." Don't shoot at a bird on the ground, cripple or otherwise. I hate to say it, but that has been something that I never assume I shouldn't have to tell someone that I'm taking along with me unless I've hunted with them a lot.
Enjoy your trip, enjoy the dog work, that is the best part!
Sorry, I added more than I thought.