Hey there, I'll take a bite to help but I still have plenty to learn. I own a das keyboard 4 and here are two reviews, one written by myself and another by a fellow GH, Dansor:
http://geekhack.org/index.php?topic=57578.0http://geekhack.org/index.php?topic=57675.0In my opinion, I think the build quality is fine and I couldn't really tell much of a difference from my previous Das keyboard (mac ver.) The version I own is blues, but assuming you need something quiet, browns should be considered. Some of the notable differences is the new design and anodized aluminum top plate. It adds a nice heft, which isn't too bad.
There are plenty of other keyboards that match your description. An example would be the Matias Quiet pro keyboard:
http://www.matias.ca/quietpro/These are well reviewed here, and a search here can produce nice results. The plus side to these is that, it's cheaper also. 173.00 USD vs. 149.95 USD. There are also other keyboards mentioned, who were in the same situation as yourself, requiring a quiet keyboard.
Besides keyboards, there have been also the use of o-rings to help dampen the sound. That can also be found by doing a quick search with nice results explained in great detail by fellow members.
Hope this helps and gives you an idea
