Yes, it is always better to find locations that are already zoned properly and developed, as it takes a lot of the risk away of losing your sign to development down the road. When you build a sign next to the dumpster pad on a property with a building on it, the odds are good that the sign will be there forever.
The rules of grandfathering are that any use that was legally built can continue after the re-zoning, but that is still subject to the interpretation of the city. I would get their take on how they perceive grandfathering to work. Even if they are legally wrong, you are probably not going to sue them over it – so you need to know up front.