1. Technically, it is not an indexed view until you create an Clustered Unique Index on the view. So you do need atleast the Clustered Unique index to call it an indexed view.
2. Beyond that, any indexing requirements are best answered by your needs. I dont think a person not familiar with your database or app can answer that part. Look up some general guidelines on what indexes need to be created. Or read more into them here (http://www.sqlservercentral.com/stairway/72399/)
3. Finally, I dont think cross database indexed views are possible.