Every poly unit works differently, I have come to find out. I am also married with a BF. I am a very honest and upfront person, so I always tell people the entire story and encourage questions upfront. Also, a rule in my relationship is that the partners need to meet as soon as it becomes "more than friends". This gets that awkwardness out of the way in the beginning, and helps the newcomer realize that everyone is actually ok with all of this.
Definitely talk to your gf about her marriage and have her explain the rules therein. Ask to meet her husband. My only red flag would be if she forbids you two to meet. Where this could be a "don't ask don't tell" type of thing, I would still insist on meeting him. But I am like that. I hate not knowing everyone, and I hate secrets.
As for long term, like they said, your chances are similar to dating anyone. But make sure to talk to her about what that would look like. Some might want long term but not be alright with bearing children together or living together. I always put that out in the beginning too, expectations. For me these are possible live-in, possible formal commitment, possible children bearing. It all depends on the people involved and what everyone decides to be the best choice.
Have courage to continue this journey, for it holds nothing but lessons and wonders.