Multiple application servers are covered in the documentation and probably best to review that first. There are options available to you that are not covered in the docs such as using multiple app servers with a load balancer or you could take a look at VMware FT technology. I dont believe the latter has been tried but technically speaking I dont see why it wouldnt work.
Remeber that the app servers pull most of the config from the DB so as long as that is HA you are quite safe, with the exception of the file store/server.
CIFS is not a supported mechanism for a windows file server, nor is DFS. Windows has no ability to mount network storage unless a user is logged in.
DFS allows you to replicate the filesystem across to another server. It works a bit like Oracle Dataguard or other DB replication tools, but on the filesystem. As such it is a local filesystem and always available even if no-one is logged in.
That could work for DR, but not HA. And it must be on a file system local to the file server. my experience w/ DFS in the past has been a network share front ending the dfs location.
if there is a single entry point for the dfs share then it may work, but that entry point must be local to the file server and cannot be a unc path.
I can't think of many good reasons to run a single app server, unless you're in a development environment or you have very few users. Multiple app servers are practically de rigeur. If your depot or your database is on network attached storage, you might consider a 10G network also. (It's quite easy to saturate a gigabit link to the depot.)