1. Yes you can add more disks later.
2. Yes, as you can read from product description. However, it is always safer to do cold-swap, if possible.
3. Certainly it is not possible if all disks are in use. If you add a new disk, there are some limited possibility to change RAID configuration (please download the manual and check it yourself)
4. "Download station" supports download from several hosting services, like rapidshare. Some of those require anyway premium accounts.
5. Probably RAID5 or RAID 6, depending on number of disks and importance of availability in the specific context.
And yes, most of those questions has been asked elsewhere. I recommend you to become friend with search button