Yes, it requires to use very fast HDDs until you finish transcoding to some non-raw format. Comparable RED raw files are from 3x to 5x smaller. Other good format for raw storage is Cineform raw, it is also much smaller and much less demanding to disk infrastructure.

Yep, you can do it. Go to http://cineform.com/, check their blog and downloads. They have conversion utility that can work with individual DNG files (Black Magic produces individual frame files). Resulting format is Cineform raw, and you can define compression ratio used. It is called DPX2CF.exe, also comes with Cineform Studio Premium.

You can also try Blackmagic Cinema Camera Cineform GUI Utility

Most probably yes. You can use either internal (with PCI-E cards) or external solutions (using USB 3.0 or Thunderbolt).

Thunderbolt based solutions

• G-Raid Thunderbold
• My Book Duo Storage

USB 3.0 solutions are usually much more affordable and you can find more options.
If you are actively shooting they will be much better approach, as you could leave drives inside bay and store it so, or just remove drives and put them into storage.
Both ways will be much cheaper than very rare Thunderbolt.

USB 3.0 based solutions

• USB 3.0 RAID multi bay solutions