Welcome to the forum! You will find there are lots of good experiences and opinions shared on this website. Some of these folks have been around a loooong time. In fact, I could believe they were around to help Noah (think Ark) dock up his boat. From hunting style to gutting techniques to pre-hunt rituals, you can find it all here. Of course, some of us are closer to our opinions than others and, as such, in some cases causes more friction than is necessary; still all good!
Keep in mind as you navigate and use this site, the final call on any topic has to be up to you; hunting is not a proxy sport. You have to get involved! There is no education like experience. Spend that time in the field, even without a weapon. Understand your quarry, learn their habits and, above all, be safe!
Now to your questions. I have purchased new and used firearms and have only had one bad experience. But, that alone would not prevent me from considering used. Heck, in some cases there are guns out there whose owner never used them; brand new condition and cheaper. Just keep your eye sharp.
One shotgun to do it all? Hmm! They are out there, but you have to then consider replacing barrels or chokes or whatever. Thompson Center immediately comes to mind. I know times are tough for many folks, however, if you could pull it off a shotgun and a rifle it may suit your needs just a little better. Remingtons are nice, so are Mossbergs and so are others; try them all for fit. Of course, you buy one then another, then another, then another.........you get the picture. Most of us suffer from this weakness!
Just my one cent worth of opinion. Remember to post pictures!