for remote installation services
and for "answer file" you should come up with the answers you need.
Mixed mode doesn't mean anything for remote installation services. What mixed mode means is that pre W2K machines can join the domain and connect without patches and updates necessary to do so.
example would be Windows 98 or Windows NT.
The unattended installation image is usually created by making an image on a clean machine, [PE images etc] then create an answer file that answers all those annoying questions that require user imput during installation. You can also include drivers and slip stream service packs as well.
All this goes into a deployment shared folder you specify for RIS installation.
I would test it first before deployment time.
Here's a technet article on adding drivers to an answer file.
http://technet.microsoft.com/en-us/library/cc766485.aspx
here's a walk through for a simple answer file.
http://technet.microsoft.com/en-us/library/cc749317.aspx
http://support.microsoft.com/kb/314460
Here's an article on how to design an answer file
http://technet.microsoft.com/en-us/library/cc738159(WS.10).aspx