Network backup setup using SD and Raid 1 enclosure

[The DataDude]

I would like to use SD to back up two PowerMac G5s over a wired network. I have purchased a WiebeTech SilverSATA II which is set up for Raid 1 mirroring of two 500gb drives:

http://www.wiebetech.com/products/silversata.php

My plan is to connect the above to one of the PowerMacs and then to share the drive on the network using personal file sharing. Here are my questions:

1. Any concerns about the setup described above?
2. Since I am backing up two separate machines, would I create two separate partitions on the backup volume and then use them as separate targets for SD's backup images? (I assume I will need to use Sparse Images).
3. Do I need to purchase two licenses for SD?
4. What about the concern that since SD doesn't have any archiving capabilities, if a file becomes corrupted at some point, SD will write over the "good" copy with a "bad" copy.
5. As the size of the backup grows and available backup disk space shrinks, does SD give some kind of warning?
6. What does one do when the size of the backup image exceeds the capacity of the backup drive? -- simply purchase a larger backup drive?

TIA!

Eric