I looked into digital signs for my church a while back.
It is actually pretty expensive to get an array of LEDs that big. Depending on how good you want the letters to look (80's video game?), you might want about 50 rows of LEDs, and proportionally I guess 100+ columns. That's 5000 bright LEDs to put in place. (And is the sign double sided?) They all have to be addressable of course, so that's a lot of wiring too. Then you need an interface to drive all of them from a computer (or chip, or microcontroller, etc.).
Once all of that is in place, the software wouldn't be too difficult.