Call me crazy. But before I spend ten million dollars or so to buy a building primarily because it has a huge wallscape facing I-10, I'd go ahead and make sure it had a permit. I wouldn't just "assume" it had one since the thing had been up for a long time.
'Cause maybe it's not, in fact, permitted. Maybe the Department of Transportation will eventually tell you to take it down. Even though you rent it out for $50,000 a month. You might assert defenses of estoppel and laches. But you might well lose in the trial court. As well as in the Court of Appeal.
The lesson -- as always -- is not to spend a lot of money on things that might well be illegal.