Import the members first: items belong to profiles, and fail if their profile doesn't exist yet. You can select all files at once: they're imported in the right order. If you do, turn off Queue Items, so the queue doesn't run them out of order.
For how names and email addresses already on your site are handled, see How Merging Works.
Run Tools > Convert Media to convert the imported audio and video. See Importer Tools. Then run an Integrity Check and reset the caches.