SharePoint 2013 Architecture Concepts Help

I'm mainly a SharePoint 2010 administrator.  Recently I've taken on a role that requires many SharePoint hats including figuring out to architect our SharePoint 2013 environment.  I'm the only one here that knows anything about SharePoint that is dedicated to this project - and while I'd like to think I know a lot, I'm not an architect.  I've been researching and researching, and I have a few questions:

- SAN? (or DAS or NAS) Is it necessary? How does this relate to the storing of databases in SQL?

- We are going to have about 25,000 users total, 7,000 concurrent, moderate use of search, and 11 department site collections (needs to be 3 content databases to segregate data for 2 departments).  Should I follow diagrams for a medium or large farm?

- I'm looking at the diagrams for a virtualized environment and they have more WFEs than the non-virtualized, why is this? Is there a reason why they have two servers on one host?

- It says 500 GB for index storage, is this per database or total?  I'm used to auto-generating content databases as you configure, I've never created the database first then assigned it to the service.  Do all the search functions go into this single 500GB databse?

Thank you so much for your help in advance! Please let me know if you'd like to know what I have for architecture so far...