It kind of depends on what you want out of a meeting. If there's something you want to publish with the Press, then you should prepare materials. You won't have to provide anything in the meeting itself - the book proposal, your CV, etc, can be done later - but have some idea about what you want to publish, what it's about, roughly how long it'll be, and so on. If you already have these proposals ready then sure, you can discuss them during the meeting itself, but unless the acquisition editor is an expert in your field (not likely), they're not going to be very useful. Besides the acquisition editor will need to get the proposal peer reviewed, so he can't decide there and then.
If there's nothing you want to publish with the Press, you don't have to prepare anything since it's up to the acquisition editor to say what he's after.
I can assure you that you don't have to be worried about this meeting because the Press likely wants you as an author (they approached you after all) so you won't be ruining any opportunities even if you show up unprepared. They can hardly expect you to be prepared if they don't tell you what to prepare beforehand after all.
Do University Press editors seek authors for chapters in an anthology? Or even just journal articles? Or are they only interested in catching the big fish?
If the Press publishes journals in your field then they're very likely to be interested in journal articles. Individual articles in an anthology are iffier, since these books still need an overall editor. If you can serve as the editor, they're likely to be interested too.
