Having done this twice in the past 4 years, my suggestion is to use rack mounted x86 PCs/servers with dual graphics cards. With ATI cards you can go to 8 or 16 monitors per server and as long as you keep a ratio of 1 screen / cpu, you should be fine (capacity wise). Using PCs (a) will allow for easy maintenance and (b) will be easy for others to work on them. PCs are also much easier to upgrade (hardware wise) as they keep the manual effort needed to a minimum. We've done this with PCs and PIs. PIs are a fun project and so far they work well, but you *will* be swearing in the process as you will have to figure out many things, including power, cabling, mounting, etc.
I built a setup like this (50X LCDs) closer to 10 yrs ago with a rack of servers, and I think it was a mistake. I should have used small desktop PCs. I was somewhat budget limited, so it was a bit of a stretch to get all the monitors driven by the limited set of servers + multiple video cards. In the end I had an array of client machines network-booting from a single server. I could have used a rack of small desktops as the clients and had 2x more CPUs and higher performance graphics cards for the same price. I would have kept an extra ~10% desktop computers on hand to swap in if any failed. Since they were network booting, there was minimal setup to add a new client to the system.