Projecting signs always make me nervous. They usually aren't too bad around here because the city limits them to 4' of projection so I haven't had to deal with something as large as the sign you're looking at. One advantage is we've been here for decades and it's a relatively small city so we know how long a structure has been in use. That at least gives you some piece of mind about how well an older structure might hold up over the long run.
Like others mentioned, first thing I'd look for is if the brackets are through bolted into some stringers on the back side of the wall. Would they have been required to get a permit to install the original structure? If so there might be some information available thru the local government. Maybe you could find out if the structure was engineered to begin with. You could also see what kind of inspections were done by the local government.
But if you've got serious doubts, listen to your intuition and walk away.
