I got this massive 165cm/16kg LED sign: https://www.alpha-american.com/p-alpha-premiere-9200c.html and it seems to be working as far as I can tell when I power it up: https://youtu.be/Msf971cHOQs . Apparently it's made to work with specific software, if you look at the manuals and specs on the alpha American site. It's a real beauty and I figured how hard can it be to run this with a pi. Well, too hard for me for sure. Look at the connections and the circuits....anyone has an idea on where to start?
You have already answered your own questions! 1) The Alpha Premiere marquee is designed such that you program the display content and format using either a purpose-built controller or custom software.
2) The manuals and catalogs that you referred to (on the Alpha American website) will tell you everything you need to know about how to control the marquee.
3) How difficult can it be to run this with a Pi? Too difficult to be worthwhile.
If you are obsessed with controlling the marquee using a Raspberry Pi, you are "doing it wrong." Controlling this thing with a Raspberry Pi will require you to dig deep into the manufacturer's technical specifications on their packet communication protocol, and then figure out how to generate the right packets with a Pi. Nasty business. Driving one with a Pi would amount to using a Pi to emulate an AMS controller. Why do that when you can just buy a second-hand controller?
If, on the other hand, you want the sign to light up and display messages before the Earth crashes into the Sun, the path is much simpler: get a copy of the Alpha Marquee ActiveX control and write your own custom code with Visual Basic. Use a Windows PC (XP or W7 is sufficient) to run your VB code. Connect the marquee to your PC's serial port using a cable purchased from Alpha American, or build the interface cable yourself--the pinout wiring is provided on their website.
A more practical but less exciting strategy is to purchase a second-hand controller (refer to the Alpha American website for model and specs) and drive the display with that.
** AMS marquees are very technical devices. I wrote programs for an AMS marquee for a few years. Dead simple with Visual Basic and the ActiveX control.
This answer probably breaks some sort of Stack Exchange rules about answering the question correctly, but you're asking about something that few people have knowledge of. I think you're better off at least knowing what you've got yourself into.