Answering my own question, +1 year later.
This may help some other folks.
AMD seemed to be the way to go; they are flexible in terms of mixing different cards together, alongside their feature rich Eyefinity system. Since this was to be used in a non-gaming, pro environment I looked into the Firepro range.
The Firepro 2270 and 2460 immediately grabbed my attention. They are cost effective, low profile, low power, passive cooled and have resp. 2 or 4 outputs. Any 2 of these cards can be used simultaneously in one seemless system. (I’m still confused about this remark, since Eyefinity alledgedly can mix up different cards anyways). The main downside of these 2270 and 2460 cards is their conservative amount of ram. I was planning to do some CAD work sooner or later, so 3D capabilities and an ample amount of ram would be necessary. Another possible issue could be that they don’t output 4k.
The W600 on the other hand has 6 miniDP outputs - each outputting up to 4k - and has 2gig’s of ram. It is pretty expensive though and has a fan. Plus it requires 6 active (or passive, not quite sure) miniDP>DVI adapters - or at least a minimum of 6 miniDP>DP adapters.
The Firepro 4100 and 5100 are less expensive alternatives, so I looked into these. They yield 4k on every output (x4) and these outputs can be further split into 1920*1080 signals by using an MST Hub. (Read the small print carefully when doing so, as far as I remember you can’t have a hub on every miniDP port.) These MST Hubs are relatively expensive by the way. The 5100 looked really attractive because of its 4gigs of ram and full sized DP connections, which would help save on bying miniDP>DP adapters. However, again the fan of these cards put me off.
Eventually, by chance I ended up bying a used “embedded” AMD E6760 card with six miniDP outputs. This was not planned, yet I couldn’t be happier. “Embedded” simply means; exactly the same as the commercial “Radeon” range, with more durable components on the side for a longer lifespan of the card. It has heatpipe cooling (and gets pretty hot), it is completely silent and it only occupies one slot. 4 of 6 of the outputs have an integrated HDMI/DVI convertor (NOT the physical connector, ONLY the embedded digital language converter chip; it’s like an integrated active adapter iow), so these outputs can work with a simple passive adapter to HDMI/DVI. When buying new, it is pretty expensive - about the same as the W600. But it’s longevity, passive cooling and form factor are real selling points. On the downside, this very card only has 1GB of ram, but we are talking about a 3year old card, new versions ar bound to have more ram. The only issue I have with these embedded cards; you can’t find much info about them. Drivers or firmware updates are nearly impossible to find - yet again that can be due to the fact that my card is 3 years old.