If you already received an offer, then it's not really an interview? But of course, all interactions with everyone in the field should be professional, so I don't see how this is different!
I think it would be a good idea to think about what you would like to work on if you accepted this offer and be ready to answer that. Also, be ready to speak about your own research. And it's great that you already have questions for this person---I would be ready to ask both scientific questions and ones about the school.
Just in case you didn't already know, it is very common for a professor who is visiting to give a talk to also meet with students in the department. Usually these meetings can span a big range of topics, from casual things like "how do you like your school?", to scientific questions about their talk topic, to general career questions like "How did you get the job you have?" etc... So it's not strange that you are meeting with the professor.
To me, I would interpret this situation as a professor is visiting a school and they know that one of the students they accepted is there so they thought it would be a good idea to meet with you. It's probably going to lean a bit more on the recruitment side, and whether or not they are recruiting for their lab specifically is not certain. I think most faculty at most departments will generally want to recruit for the good of the department, so this person would still talk to you even if they had no space in their lab.
But, I think you are overthinking this! It is a meeting with a professor at a school you have been accepted to and maybe doing research you're interested in. In general, you would want to have some good scientific questions for any scientist you meet. If you want to join this lab specifically, then ask about that. Otherwise, I would also ask about the graduate program in general at this school. And relax, it's just a conversation/meeting---you're not committing yourself to anything at this point.