Build Sharepoint based extranet

We need to create individual sharepoint based extranets for invited guests only. User authentication to be handled as Guest Users in Active Directory, with One Time Passcode instead of passwords. Our guest users sit behind their employers' corporate firewalls (no chance of creating trusted machine status). Guests need to read and accept a terms and conditions document before access is provided. Would prefer a customised email invitation to avoid the email landing in junk mail, but if it is easier then will accept MS default. Guests should not be able to download or copy/paste anything, but simply view pages