I think given the fairly low cost of servers you should start with 2 Physical servers, even if the second one is the cheapest you can find. You will appreciate the flexibility / redundancy.
If you have a large array say 6TB or more then you will want RAID6 as the chance of another disk failing while replacing the faulty disk is just too high!
I would be interested in everyones thoughts on the best method but in the short term, join the backup server to the domain so that replication can occur and use robocopy or something to sync the data. You could also setup DFS on any shares.