Ok, it does look like you can move your disk pack (all disks) to another Drobo. It stipulates all disks must be present, and if you buy a 5N it has to be to another 5N. If one of your disks fails during the Drobos failure, it looks like you will not be able to migrate. Thats new to me!
http://www.drobo.com/products/migrations.php
http://support.drobo.com/app/answers/detail/a_id/274/~/can-i-move-my-disk-pack-from-one-drobo-storage-device-to-another%3F
Drobo uses a proprietary system called BeyondRaid. Any data stored on the disks will show up as .tib (database I assume) files rather than the original files. It is unlikely that recovery software will be able to do anything with these files.
Getting 15TB is going to be expensive...
If you put 5 4TB drives in it, that will give you 14.5 TB of storage. If you switch on dual disk redundancy, you will get about 11TB of storage.
http://www.drobo.co.uk/products/capacity-calculator/
A Drobo 5N with 5X 4TB drives would cost you £1449 to get 14.5TB Single disk redundancy or 11TB Dual disk redundancy.
A Drobo 5N with 5X 3TB drives would cost you £950 to get 11TB Single disk redundancy.
Of course, the great thing about Drobos is that you can put in any disks you have lying around, and grow the disk pack asymmetrically as you need to.
All that being said, I have found my Drobo to be rock solid. It has never lost any data, even when it has spat dead drives out at me. It's been ticking along since 2009, and hooked up with Firewire 800 its plenty quick enough for video editing. It is very fuss free as well, I like kit that lets me get on with what I'm doing without me having to faff with it