Wow, the EPROM suggestion sounds great! At least you've given us a ballpark figure as far as how much data can be stored plus the suggestion for the real-time clock.
How close can you get? Those things don't have a great range. Or can you access an Internet point somewhere?
Uh, this is probably the most prickly of the challenges. We are going to be able to get fairly close - like "out in front of the building" close. But since the system is going to be up on the roof, we probably won't have much line of sight... although if it could really keep the price down, perhaps we could mount it on the street facing wall -- hmmm.
Thanks for all your thoughts. As you can probably tell by now we know very little of electronics, but if we can get the hardware figured out, we can find someone locally who could probably put the items together and set up the program/sketch. (Why do people need to invent another word for "program", i wonder).
Have a wonderful evening!