Hello,
1. I bought it simply at Ebay. In Germany,, the prices seem to be lower. Look for Futro S920, it should show up. (when shipping to Estonia is an option)
Don't buy the cheaper S720 or S520.
2. Correct, both SATA ports work at the same time. You can get a msata to sata adapter card from chine for lower than 5€. With DSM/Xpenology, the OS is installed on all available HDDs, no choice, as much as I know.
3. I am running a Raid 1 system with two 931.5 GB drives. I get 921GB Space, therfore, DSM took about 10GB.
Regards, Tomte