Okay, let’s see. Where are we then?
You don’t really need any kind of RAID setup… but it sounds like you want one. Of the various configurations suggested, it sounds like RAID1 is your first choice for relative simplicity, fewer drives, and lowest cost. (If lowest cost is the primary factor above all others, then consider dropping RAID1 altogether. It’s just doubling your disk cost and not providing any worthwhile performance or reliability improvement).
Then I think we’ve crossed over into backup strategies a bit as well. A recommended best practice, as explained by jwz, is to employ both local and remote backups. For remote backups,we’re somewhat concerned about the throughput rate if using network bandwidth to move files. You estimated you could copy 8GB in “well under a week”. Let’s estimate you can schlep about 2GB of data over the wire per day. And let’s also figure that you might need to someday backup a sizable portion of one of those 750GB drives… guess oh, about 300GB worth. You’d be looking at 150 days (running 24/7 without interruption) to accomplish the task. That’s not a realistically supportable strategy.
If getting a better uplink isn’t possible (or financially responsible), then I suggest you clone the content of the 750GB drive on a regular basis… maybe once a month or every other week. Like for the local backup, you could use rsync for that task as well. And then keep that clone somewhere off site in case lightning strikes, or a flood hits, or some other unexpected event removes your server and primary disks from service.