How do I combine Wordpress sites into a multisite?

There is a task to combine several different sites into a multisite:

1. The problem is that all of them with their own set of plugins and themes, different versions of WP
2. I still don't understand what to do with the database. How to combine them and connect to the multisite. Just there is a nuance that the base, there are large (sites made by designers, and some of them are 200-300 Mb), there are with different prefixes.

How to unite all this and whether it makes sense in terms of speed and space on hosting.