1) No, not if you aren't doing an upgrade.
2) Not sure what you mean by "Network Users"? If you have "USERA" that is using "CLIENTA" to access shared files on "SERVERA" you are also going to create local user named "USERA" on the file server with the same password so the authentication will pass through.
3) There shouldn't be any limits.
I am also curious as to why you aren't using a domain environemnt? They are easy to set up and would make file sharing / permissions much easier to manage.