To my knowledge, a seperate Indexing server is not a best practice. If you did that, you would have the Storage service and Indexing service on seperate Vault Servers. This would create a lot of MSMQ traffic between servers. In my experience, the best performance is gained by having the storage and indexing service for an archive on the same server.
You do need high-performing SCSI, FC attached storage for indexes.
Also, don't worry about the number of servers, EV is sold my the Mailbox/User