A bit of advice. There's a bit more legwork you'll need to do, however, I don't have time today to spell it all out today. I checked out that "link" - it's not "promising" at all for Mac users, and they're leaving out quite a bit of relevant information, mainly which refresh rate you'll be looking at.
Also, regarding Matrox's products for multiple displays, the subject Mac is likely running Yosemite - the only eGPU units that Matrox sells that supports Macs and Yosemite only supports DVI-D displays, a potential limitation, and will require the use of a USB port from somewhere to power the eGPU.
First, check on the displays you'll want to use - I have 2 Dell P2715Q displays connected to my late-2013 rMBP; I can drive each at 4k 60Hz over two separate mDP>DP cables, but to "daisy chain" them they'll have to be set up for MST and they'll only work at 30Hz. Not many - if any - Mac laptops will supply sufficient graphics power to feed 3 daisy chained displays, and will require at least 10.10.3 installed on the subject Mac.
The only other displays that could/would smoothly enough are Thunderbolt displays, either by Apple or LG - the TB interface in the displays properly handles the mixing/demuxing of the video signals that HDMI, DVI-D, and DP (which is carrying an HDMI signal) choke on. The second display in that DP daisy chain is terminating that HDMI signal; a DP display following a TB display will terminate that video signal.
A tip, from a short slideshow I gave just this past Friday when my brother was wanting to show his pictures from his trip to HK and South Africa. The pics were shown on my rMBP screen, and mirrored to each of the two Dell displays, and my Sony Bravia TV that's connected to an Apple TV; my Dells are on Herman Miller arms and allowed me to rotate the Dells to face some of the people in the room. So, there were four displays in action there - all in "HD" (not 4k), and it worked well and only took about 2-3 minutes to get going.
Another tip, get cables with the best throughput you can find. I finally settled on Lindy and StarTech DP 1.2 2160p cables - some will say DP is DP, and I'll offer a "you don't know what you're blathering about...". I have a box of 1440p and 1600p cables that leave our workspaces looking laggy.
If your client wants to impress, I read this just this morning:
http://9to5mac.com/2015/08/31/mac-pro-4k-monitors-six/