Multiple installations vs Multisite vs Single installation

I’m going to be starting a project soon for a non-profit where they are currently using multiple individual WordPress installations.

1) For their main site, where they require event-management plugins, have multiple custom post types, and lots of content.
2) A separate site for their annual conference (which uses an entirely different style/theme).
3) A separate installation for a pre-existing blog, but which uses the same theme as (1) the main site. It’s designed to look as if it were still the same site.
4) They’re wanting to open up an online store to sell tshirts, books, etc.

My question is, which option might be my best? Integrating all of these different arms into 1 WordPress installation (with a blog, an online store, and a section for their annual conference).
Would it be best to create a multisite Network, and manage these different sites there?
Or continue to have these be separate installations?

Any help would be greatly greatly appreciated!